check for the pattern length after parsing the full argument in pkg info.

Checking the pattern length before parsing it resulted in passing an
empty string to the pkgdb_query() function in some corner cases,
triggering an assert.
Removed an noop codepath.

the `pkgversion' is already initialized to NULL when declared, the
useless else branch was confusing.
Improve consistancy of the "Pattern must not be empty" message.

The same message was used in both search and info but with different
punctuation. While the search message version was more enthusiastic it
seems very unfair for good ol' info.