[oracle] Fix handling of NULL values when add features to provider

Fixes NULL attribute values are incorrectly converted to empty
strings or 0 numeric values instead of NULL.

(cherry-picked from befed4d)

Author: nyalldawson

src/providers/oracle/qgsoracleprovider.cpp

@@ -1370,11 +1370,14 @@ bool QgsOracleProvider::addFeatures( QgsFeatureList &flist )
         QVariant value = attributevec[ fieldId[i] ];
 
         QString v;
-        if ( !value.isValid() )
+        if ( !value.isValid() || value.isNull() )
         {
-          const QgsField &fld = field( fieldId[i] );
-          v = paramValue( defaultValues[i], defaultValues[i] );
-          features->setAttribute( fieldId[i], convertValue( fld.type(), v ) );
+          if ( mPrimaryKeyAttrs.contains( i ) && !defaultValues.at( i ).isEmpty() )
+          {
+            const QgsField &fld = field( fieldId[i] );
+            v = paramValue( defaultValues[i], defaultValues[i] );
+            features->setAttribute( fieldId[i], convertValue( fld.type(), v ) );
+          }
         }
         else
         {