feat: create empty Table without schema (#128)

### Summary of Changes

We no longer raise an exception when a user creates an empty `Table`
without specifying a schema. `Table([])` is now allows.

`add_row` and `add_rows` must still be adjusted (#127) to allow adding
rows to such an empty `Table`.

---------

Co-authored-by: lars-reimann <lars-reimann@users.noreply.github.com>

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

@@ -18,7 +18,6 @@
     DuplicateColumnNameError,
     IndexOutOfBoundsError,
     MissingDataError,
-    MissingSchemaError,
     NonNumericColumnError,
     SchemaMismatchError,
     UnknownColumnNameError,
@@ -193,14 +192,10 @@ def from_rows(rows: list[Row]) -> Table:
 
     def __init__(self, data: Iterable, schema: Optional[TableSchema] = None):
         self._data: pd.Dataframe = data if isinstance(data, pd.DataFrame) else pd.DataFrame(data)
-        if schema is None:
-            if self.count_columns() == 0:
-                raise MissingSchemaError()
-            self._schema: TableSchema = TableSchema._from_dataframe(self._data)
-        else:
-            self._schema = schema
-            if self._data.empty:
-                self._data = pd.DataFrame(columns=self._schema.get_column_names())
+        self._schema: TableSchema = TableSchema._from_dataframe(self._data) if schema is None else schema
+
+        if self._data.empty:
+            self._data = pd.DataFrame(columns=self._schema.get_column_names())
 
         self._data = self._data.reset_index(drop=True)
         self._data.columns = list(range(self.count_columns()))

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

@@ -4,10 +4,13 @@
 
 
 def test_create_empty_table() -> None:
-    table = Table(
-        [], TableSchema({"col1": ColumnType.from_numpy_dtype(np.dtype(float))})
-    )
+    table = Table([], TableSchema({"col1": ColumnType.from_numpy_dtype(np.dtype(float))}))
     col = table.get_column("col1")
     assert col.count() == 0
     assert isinstance(col.type, type(ColumnType.from_numpy_dtype(np.dtype(float))))
     assert col.name == "col1"
+
+
+def test_create_empty_table_without_schema() -> None:
+    table = Table([])
+    assert table.schema == TableSchema({})