In particular, since commit 8eb7f6df, even if it returns an empty
array of procedure, it still means an array of size 1 (with the unique
value being NULL).

This also fixes the public API gimp_pdb_query_procedures() which returns
a GStrv using gimp_pdb_query() internally.

Finally don't keep track of the array size, just recompute it with
g_strv_length(), which will be much less bug-prone.