glib-aux: fix nicks for zero flag in nm_utils_enum_to_str()

nm_utils_enum_to_str() can print flags, that is, combinations of
powers of two integers.

It also supports nicks, for certain flags.

When we have a nick for value zero, then that requires special
handling. Otherwise, that zero nick will always show up in the
string representation, although, it should only be used if the
enum value is exactly zero.
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