Add 'not selected' option for optional layer parameters in processing…

… model view

Fixes #19329 (https://issues.qgis.org/issues/19329)

Optional layer parameters in processing algorithms used in the graphical modeler
add an empty string as the default parameter value, which can raise an exception
during algorithm execution. This update replaces the default empty string with a
default 'Not selected' option.

python/plugins/processing/gui/wrappers.py

@@ -864,10 +864,10 @@ def createWidget(self):
             self.combo = QComboBox()
             layers = self.getAvailableLayers()
             self.combo.setEditable(True)
+            if self.param.flags() & QgsProcessingParameterDefinition.FlagOptional:
+                self.combo.addItem(self.NOT_SELECTED, self.NOT_SET_OPTION)
             for layer in layers:
                 self.combo.addItem(self.dialog.resolveValueDescription(layer), layer)
-            if self.param.flags() & QgsProcessingParameterDefinition.FlagOptional:
-                self.combo.setEditText("")
 
             widget = QWidget()
             layout = QHBoxLayout()