qgspostgresprovider: Fix primary key retrieval with lowercase option

The `lowercaseFieldNames` allows to transform the column names to
lowercase. If this option is enabled, then the requested primary
key (`primaryKey`) is in lowercase but the `fields` are
not. Therefore, the requested primary key will never be found.

This issues is fixed by converting the field names to lowercase when
looking for the primary key if the option is enabled.

Closes: qgis#55856

src/providers/postgres/qgspostgresprovider.cpp

@@ -4609,13 +4609,15 @@ Qgis::VectorExportResult QgsPostgresProvider::createEmptyLayer( const QString &u
   {
     pkList = parseUriKey( primaryKey );
     const auto constPkList = pkList;
+    const bool lowercaseFieldNames = options && options->value( QStringLiteral( "lowercaseFieldNames" ), false ).toBool();
     for ( const QString &col : constPkList )
     {
       // search for the passed field
       QString type;
       for ( int fldIdx = 0; fldIdx < fields.count(); ++fldIdx )
       {
-        if ( fields[fldIdx].name() == col )
+        const QString fieldName = lowercaseFieldNames ? fields[fldIdx].name().toLower() : fields[fldIdx].name();
+        if ( fieldName == col )
         {
           // found, get the field type
           QgsField fld = fields[fldIdx];