[processing] fix map layer combo box layer reference

Author: nirvn

python/plugins/processing/gui/wrappers.py

@@ -763,7 +763,7 @@ def value(self):
         if self.dialogType == DIALOG_STANDARD:
             try:
                 layer = self.combo.currentLayer()
-                if layer:
+                if layer is not None:
                     return layer
                 else:
                     return self.combo.currentText()
@@ -1226,7 +1226,7 @@ def value(self):
         if self.dialogType == DIALOG_STANDARD:
             try:
                 layer = self.combo.currentLayer()
-                if layer:
+                if layer is not None:
                     return layer
                 else:
                     return self.combo.currentText()