Fix QgsFields::lookupField returns bad result for empty field name

QString::isNull vs ::isEmpty strikes again!

src/core/qgsfields.cpp

@@ -290,6 +290,9 @@ QIcon QgsFields::iconForField( int fieldIdx ) const
 
 int QgsFields::lookupField( const QString& fieldName ) const
 {
+  if ( fieldName.isEmpty() ) //shortcut
+    return -1;
+
   for ( int idx = 0; idx < count(); ++idx )
   {
     if ( d->fields[idx].field.name() == fieldName )
@@ -305,7 +308,7 @@ int QgsFields::lookupField( const QString& fieldName ) const
   for ( int idx = 0; idx < count(); ++idx )
   {
     QString alias = d->fields[idx].field.alias();
-    if ( !alias.isNull() && QString::compare( alias, fieldName, Qt::CaseInsensitive ) == 0 )
+    if ( !alias.isEmpty() && QString::compare( alias, fieldName, Qt::CaseInsensitive ) == 0 )
       return idx;
   }
 

tests/src/core/testqgsfields.cpp

@@ -335,8 +335,12 @@ void TestQgsFields::indexFromName()
   QgsField field2( QStringLiteral( "testfield2" ) );
   fields.append( field2 );
   QgsField field3( QStringLiteral( "testfield3" ) );
+  field3.setAlias( QStringLiteral( "" ) );
   fields.append( field3 );
 
+  QCOMPARE( fields.lookupField( QString( "" ) ), -1 );
+  QCOMPARE( fields.lookupField( QString() ), -1 );
+
   QCOMPARE( fields.indexFromName( QString( "bad" ) ), -1 );
   QCOMPARE( fields.lookupField( QString( "bad" ) ), -1 );
   QCOMPARE( fields.indexFromName( QString( "testfield" ) ), 0 );