[BUGFIX] Add condition before converting FlexForm to Array

There is a special case, when saving a new record in the BE,
variable $row[$fieldName] is already an array and must not
be processed by the configuration service. Otherwise, a
warning is raised, telling:

PHP Warning: substr() expects parameter 1 to be string, array given.
Merge pull request #463 from fudriot/bugfix-conditional-flexform-conversion

[BUGFIX] Add condition before converting FlexForm to Array