Check if the revision property is valid before using it
Check if the revision property is valid before using it

The revision property is set to an empty string when the build was
initiated by a trigger. As the initial value of merge_sha is also an
empty string, an arbitrary PR is mistakenly chosen, leading to a
mysterious "test successful" message. We should check if the property is
valid before utilising it.

Fixes #15.

coupa/homu,frewsxcv/homu,karen-irc/homu,servo/homu,coldnight/homu,barosl/homu,mvines/homu,tempbottle/homu,acmcarther/homu,karen-irc/homu,acmcarther/homu,coupa/homu,coldnight/homu,tempbottle/homu,frewsxcv/homu,servo/homu,fizzy123/homu,mvines/homu,fizzy123/homu,barosl/homu