fix: Warnings raised by underlying seaborn and numpy libraries (#425)

Closes #357 

### Summary of Changes

* Python's `warnings` module now ignores warnings caused by underlying
seaborn and numpy libraries.

---------

Co-authored-by: megalinter-bot <129584137+megalinter-bot@users.noreply.github.com>
Co-authored-by: Alexander <47296670+Marsmaennchen221@users.noreply.github.com>

src/safeds/data/tabular/containers/_table.py

@@ -1808,16 +1808,41 @@ def plot_correlation_heatmap(self) -> Image:
         """
         only_numerical = self.remove_columns_with_non_numerical_values()
 
-        fig = plt.figure()
-        sns.heatmap(
-            data=only_numerical._data.corr(),
-            vmin=-1,
-            vmax=1,
-            xticklabels=only_numerical.column_names,
-            yticklabels=only_numerical.column_names,
-            cmap="vlag",
-        )
-        plt.tight_layout()
+        if self.number_of_rows == 0:
+            warnings.warn(
+                "An empty table has been used. A correlation heatmap on an empty table will show nothing.",
+                stacklevel=2,
+            )
+
+            with warnings.catch_warnings():
+                warnings.filterwarnings(
+                    "ignore",
+                    message=(
+                        "Attempting to set identical low and high (xlims|ylims) makes transformation singular;"
+                        " automatically expanding."
+                    ),
+                )
+                fig = plt.figure()
+                sns.heatmap(
+                    data=only_numerical._data.corr(),
+                    vmin=-1,
+                    vmax=1,
+                    xticklabels=only_numerical.column_names,
+                    yticklabels=only_numerical.column_names,
+                    cmap="vlag",
+                )
+                plt.tight_layout()
+        else:
+            fig = plt.figure()
+            sns.heatmap(
+                data=only_numerical._data.corr(),
+                vmin=-1,
+                vmax=1,
+                xticklabels=only_numerical.column_names,
+                yticklabels=only_numerical.column_names,
+                cmap="vlag",
+            )
+            plt.tight_layout()
 
         buffer = io.BytesIO()
         fig.savefig(buffer, format="png")
@@ -2000,7 +2025,7 @@ def plot_histograms(self) -> Image:
         """
         col_wrap = min(self.number_of_columns, 3)
 
-        data = pd.melt(self._data, value_vars=self.column_names)
+        data = pd.melt(self._data.applymap(lambda value: str(value)), value_vars=self.column_names)
         grid = sns.FacetGrid(data=data, col="variable", col_wrap=col_wrap, sharex=False, sharey=False)
         grid.map(sns.histplot, "value")
         grid.set_xlabels("")

tests/safeds/data/tabular/containers/_table/test_plot_correlation_heatmap.py

@@ -14,9 +14,16 @@
     ids=["normal", "empty"],
 )
 def test_should_match_snapshot(table: Table, path: str) -> None:
-    current = table.plot_correlation_heatmap()
+    if table.number_of_rows == 0:
+        with pytest.warns(
+            UserWarning,
+            match=r"An empty table has been used. A correlation heatmap on an empty table will show nothing.",
+        ):
+            current = table.plot_correlation_heatmap()
+    else:
+        current = table.plot_correlation_heatmap()
     snapshot = Image.from_png_file(resolve_resource_path(path))
 
     # Inlining the expression into the assert causes pytest to hang if the assertion fails when run from PyCharm.
-    assertion = snapshot._image.tobytes() == current._image.tobytes()
+    assertion = snapshot == current
     assert assertion

tests/safeds/data/tabular/containers/_table/test_plot_histograms.py

@@ -10,7 +10,14 @@
     [
         (Table({"A": [1, 2, 3]}), "./image/snapshot_histograms/one_column.png"),
         (
-            Table({"A": [1, 2, 3], "B": ["A", "A", "Bla"], "C": [True, True, False], "D": [1.0, 2.1, 4.5]}),
+            Table(
+                {
+                    "A": [1, 2, 3, 3, 2, 4, 2],
+                    "B": ["a", "b", "b", "b", "b", "b", "a"],
+                    "C": [True, True, False, True, False, None, True],
+                    "D": [1.0, 2.1, 2.1, 2.1, 2.1, 3.0, 3.0],
+                },
+            ),
             "./image/snapshot_histograms/four_columns.png",
         ),
     ],
@@ -19,9 +26,8 @@
 def test_should_match_snapshot(table: Table, path: str) -> None:
     current = table.plot_histograms()
     snapshot = Image.from_png_file(resolve_resource_path(path))
-
     # Inlining the expression into the assert causes pytest to hang if the assertion fails when run from PyCharm.
-    assertion = snapshot._image.tobytes() == current._image.tobytes()
+    assertion = snapshot == current
     assert assertion