[pal] fix curved labels failure with zero-width characters

(fixes 15801)

Author: nirvn

src/core/pal/feature.cpp

@@ -1059,6 +1059,10 @@ LabelPosition* FeaturePart::curvedPlacementAtOffset( PointSet* path_positions, d
 
     // grab the next character according to the orientation
     LabelInfo::CharacterInfo& ci = ( orientation > 0 ? li->char_info[i] : li->char_info[li->char_num-i-1] );
+    if ( qgsDoubleNear( ci.width, 0.0 ) )
+      // Certain scripts rely on zero-width character, skip those to prevent failure (see #15801)
+      continue;
+
     double start_x, start_y, end_x, end_y;
     if ( nextCharPosition( ci.width, path_distances[index], path_positions, index, distance, start_x, start_y, end_x, end_y ) == false )
     {

tests/src/python/test_qgspallabeling_placement.py

@@ -433,6 +433,18 @@ def test_label_line_avoid_jaggy(self):
         self.removeMapLayer(self.layer)
         self.layer = None
 
+    def test_label_curved_zero_width_char(self):
+        # Test that curved label work with zero-width characters
+        self.layer = TestQgsPalLabeling.loadFeatureLayer('line')
+        self._TestMapSettings = self.cloneMapSettings(self._MapSettings)
+        self.lyr.placement = QgsPalLayerSettings.Curved
+        self.lyr.placementFlags = QgsPalLayerSettings.OnLine
+        self.lyr.fieldName = "'invisible​space'"
+        self.lyr.isExpression = True
+        self.checkTest()
+        self.removeMapLayer(self.layer)
+        self.layer = None
+
 if __name__ == '__main__':
     # NOTE: unless PAL_SUITE env var is set all test class methods will be run
     # SEE: test_qgspallabeling_tests.suiteTests() to define suite

tests/src/python/test_qgspallabeling_tests.py

@@ -308,8 +308,9 @@ def test_curved_placement_below(self):
         self.lyr.placementFlags = QgsPalLayerSettings.BelowLine | QgsPalLayerSettings.MapOrientation
         self.checkTest()
 
-
 # noinspection PyPep8Naming
+
+
 def suiteTests():
     """
     Use to define which tests are run when PAL_SUITE is set.