MDL-4207 Don't overwrite modified user profile values with empty LDAP values when...

MDL-4207 Don't overwrite modified user profile values with empty LDAP values when 'unlocked if empty' is set

If user profile values are locked to LDAP, then LDAP should provide those
values.

But the purpose of the setting 'unlocked if empty' is to allow the user to fill
in a value for the selected field _if LDAP is giving nothing_ for this field.
Thus it makaes sense to let this value stand in until LDAP is giving a value
for this field.

Merged from MOODLE_18_STABLE