Don't silently discard requests to set QgsFieldExpressionWidget

to an empty expression

This causes issues in lots of places - e.g. a field constraint
is always set to the first field in a layer, and cannot be
removed.

I've looked at all calls to setField and without exception
they should accept that setting the field to an empty string
results in an empty expression.

Fixes #14325

src/gui/qgsfieldexpressionwidget.cpp

@@ -173,7 +173,10 @@ void QgsFieldExpressionWidget::setLayer( QgsMapLayer *layer )
 void QgsFieldExpressionWidget::setField( const QString &fieldName )
 {
   if ( fieldName.isEmpty() )
+  {
+    setRow( -1 );
     return;
+  }
 
   QModelIndex idx = mFieldProxyModel->sourceFieldModel()->indexFromName( fieldName );
   if ( !idx.isValid() )

tests/src/gui/testqgsfieldexpressionwidget.cpp

@@ -47,6 +47,7 @@ class TestQgsFieldExpressionWidget : public QObject
     void asExpression();
     void testIsValid();
     void testFilters();
+    void setNull();
 
   private:
     QgsFieldExpressionWidget *mWidget = nullptr;
@@ -269,6 +270,27 @@ void TestQgsFieldExpressionWidget::testFilters()
   QgsProject::instance()->removeMapLayer( layer );
 }
 
+void TestQgsFieldExpressionWidget::setNull()
+{
+  // test that QgsFieldExpressionWidget can be set to an empty value
+  QgsVectorLayer *layer = new QgsVectorLayer( QStringLiteral( "point?field=fld:int&field=fld2:int&field=fld3:int" ), QStringLiteral( "x" ), QStringLiteral( "memory" ) );
+  QgsProject::instance()->addMapLayer( layer );
+
+  std::unique_ptr< QgsFieldExpressionWidget > widget( new QgsFieldExpressionWidget() );
+  widget->setLayer( layer );
+
+  widget->setField( QString() );
+  QVERIFY( widget->currentField().isEmpty() );
+
+  widget->setField( QStringLiteral( "fld2" ) );
+  QCOMPARE( widget->currentField(), QStringLiteral( "fld2" ) );
+
+  widget->setField( QString() );
+  QVERIFY( widget->currentField().isEmpty() );
+
+  QgsProject::instance()->removeMapLayer( layer );
+}
+
 QGSTEST_MAIN( TestQgsFieldExpressionWidget )
 #include "testqgsfieldexpressionwidget.moc"