extcap: Remove single-space argument for boolflags

 Boolflags will be added with a NULL argument to the
 list and therefore the original fix will work.

 Note: extcap_get_complex_as_string will convert any NULL
 value into a single character argument (' '). Therefore
 any argument type, where NULL is a special value should
 be handled seperately here. For now, the only datatype
 will be BOOLFLAG

Change-Id: Icfc9b573214e05ea713a3b70ea37953d470dafae
Reviewed-on: https://code.wireshark.org/review/4649
Reviewed-by: Roland Knall <rknall@gmail.com>
Reviewed-by: Tomasz Moń <desowin@gmail.com>
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>