Fix value map search widget doesn't work for null/not null searches

before an initial value is picked

Refs #17986

Author: nyalldawson

src/gui/editorwidgets/qgsvaluemapsearchwidgetwrapper.cpp

@@ -80,10 +80,6 @@ QgsSearchWidgetWrapper::FilterFlags QgsValueMapSearchWidgetWrapper::defaultFlags
 
 QString QgsValueMapSearchWidgetWrapper::createExpression( QgsSearchWidgetWrapper::FilterFlags flags ) const
 {
-  //if deselect value, always pass
-  if ( mComboBox->currentIndex() == 0 )
-    return QString();
-
   //clear any unsupported flags
   flags &= supportedFlags();
 
@@ -95,6 +91,10 @@ QString QgsValueMapSearchWidgetWrapper::createExpression( QgsSearchWidgetWrapper
   if ( flags & IsNotNull )
     return fieldName + " IS NOT NULL";
 
+  //if deselect value, always pass
+  if ( mComboBox->currentIndex() == 0 )
+    return QString();
+
   QString currentKey = mComboBox->currentData().toString();
 
   switch ( fldType )

tests/src/python/test_qgssearchwidgetwrapper.py

@@ -142,8 +142,8 @@ def testCreateExpression(self):
         # first, set it to the "select value" item
         c.setCurrentIndex(0)
 
-        self.assertEqual(w.createExpression(QgsSearchWidgetWrapper.IsNull), '')
-        self.assertEqual(w.createExpression(QgsSearchWidgetWrapper.IsNotNull), '')
+        self.assertEqual(w.createExpression(QgsSearchWidgetWrapper.IsNull), '"fldtxt" IS NULL')
+        self.assertEqual(w.createExpression(QgsSearchWidgetWrapper.IsNotNull), '"fldtxt" IS NOT NULL')
         self.assertEqual(w.createExpression(QgsSearchWidgetWrapper.EqualTo), '')
         self.assertEqual(w.createExpression(QgsSearchWidgetWrapper.NotEqualTo), '')