util: Fix gerrit bot commit subject parser
util: Fix gerrit bot commit subject parser

Currently, if the commit subject does have tags, the parser will
return the list of tags as a NoneType object, which will be
iterated later. This caused the gerrit bot to fail.

This change lets the parser return the list of tags as an empty
list rather than a NoneType object. Also, a commit subject without
a semicolon `:` will be assumed as having no tags and the whole
subject is the commit message.

Signed-off-by: Hoa Nguyen <a0aa8449aacb303edcb25d6bbe69da9124f010fc@ucdavis.edu>
Change-Id: Ie8c90e14bb85c4ce1c583121d02e75aa87db7811
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/48586
Reviewed-by: Bobby R. Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Maintainer: Bobby R. Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5