There is only one implementation now.

plugins: imap-filter-sieve: cmd-filter-sieve - Avoid using imap_arg_as_nstring() in cmd_filter_sieve_script_parse_value_arg()

The argument can only be a quoted string here, so imap_arg_as_string() will suffice.

This was not supposed to be accepted and led to unexpected behavior:

- FILTER SIEVE PERSONAL NIL was handled as FILTER SIEVE DELIVERY,
- FILTER SIEVE GLOBAL NIL caused an internal error.

Fixes assert failure occurring either at compiletime or at runtime (depending on
whether the value originates from variable).

Fixes assert failure occurring either at compiletime or at runtime (depending on
whether the value originates from variable).

Fixes assert failure occurring at runtime (when value originates from variable).