useradd-staticids.bbclass: Fix for Bug 6633

When using the useradd-staticids.bbclass under meta/classes,
this error occurs:
"<username> - <username>: Username  does not have a static uid defined."
There was a problem with the regular expression for parsing parameters,
it was sometimes returning an empty string.

I have fixed this by skipping empty strings.

(From OE-Core rev: f249ef32709069a2680b92dc5a5b4f6545d014b7)

Signed-off-by: Fabrice Coulon <fabrice@axis.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>