[BUGFIX] Use raw flux field name where required and NULL must be avoided

This fixes an issue on 4.x branches when retrieving flexform variables; before this patch the internal getFieldName() was being used which would return NULL on 4.x branches, making getFlexFormValues() return too early.
Merge pull request #163 from NamelessCoder/rawfieldname

[BUGFIX] Use raw flux field name where required and NULL must be avoided