avoid reloading fields when existing provider encoding is re-set (fixes #10340)

Author: jef-n

src/core/qgsvectorlayer.cpp

@@ -583,7 +583,7 @@ const QgsVectorDataProvider* QgsVectorLayer::dataProvider() const
 
 void QgsVectorLayer::setProviderEncoding( const QString& encoding )
 {
-  if ( mDataProvider )
+  if ( mDataProvider && mDataProvider->encoding() != encoding )
   {
     mDataProvider->setEncoding( encoding );
     updateFields();