vector layer: ignore non-existing fields in min/max/uniqueValues (fixes #13194)

(cherry picked from commit 71e7aab)

Author: jef-n

src/core/qgsvectorlayer.cpp

@@ -2955,6 +2955,10 @@ void QgsVectorLayer::uniqueValues( int index, QList<QVariant> &uniqueValues, int
   }
 
   QgsFields::FieldOrigin origin = mUpdatedFields.fieldOrigin( index );
+  if ( origin == QgsFields::OriginUnknown )
+  {
+    return;
+  }
 
   if ( origin == QgsFields::OriginProvider ) //a provider field
   {
@@ -3045,6 +3049,10 @@ QVariant QgsVectorLayer::minimumValue( int index )
   }
 
   QgsFields::FieldOrigin origin = mUpdatedFields.fieldOrigin( index );
+  if ( origin == QgsFields::OriginUnknown )
+  {
+    return QVariant();
+  }
 
   if ( origin == QgsFields::OriginProvider ) //a provider field
   {
@@ -3107,6 +3115,10 @@ QVariant QgsVectorLayer::maximumValue( int index )
   }
 
   QgsFields::FieldOrigin origin = mUpdatedFields.fieldOrigin( index );
+  if ( origin == QgsFields::OriginUnknown )
+  {
+    return QVariant();
+  }
 
   if ( origin == QgsFields::OriginProvider ) //a provider field
   {