Fix TestingServer handling of missing buildnumber parameter.

Surprisingly (undocumented-ly), webapp2.Request.get(param) returns
empty string (rather than None) when a parameter is missing.

R=ojan@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/414123002

git-svn-id: svn://svn.chromium.org/blink/trunk@178878 bbb929c8-8fbe-4397-9dbb-9b2b20218538