Handle GN/MB analyze crash when running a tryjob with no modified files.
Handle GN/MB analyze crash when running a tryjob with no modified files.

We've seen some try jobs launched via the CQ where experimental bots
get delayed and don't run until after the CL lands; in that situation,
the patch applies to the bot but no files are considered 'modified',
and so when we run 'gn analyze' there are no files to analyze and
GN errs out.

This change makes things a bit more robust so that we print a warning
and exit early with 'no compile necessary' instead.

A better fix might be to determine the list of affected files
differently, but it's unclear how important that is.

R=phajdan.jr@chromium.org
BUG=511441

whitespace change

Review URL: https://codereview.chromium.org/1244533005

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#339986}

Chilledheart/chromium,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Chilledheart/chromium,Chilledheart/chromium,axinging/chromium-crosswalk,Just-D/chromium-1,Just-D/chromium-1,axinging/chromium-crosswalk,axinging/chromium-crosswalk,Just-D/chromium-1,Just-D/chromium-1,Pluto-tv/chromium-crosswalk,Just-D/chromium-1,Chilledheart/chromium,axinging/chromium-crosswalk,Chilledheart/chromium,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Chilledheart/chromium,Just-D/chromium-1,axinging/chromium-crosswalk,Chilledheart/chromium,Pluto-tv/chromium-crosswalk,Chilledheart/chromium,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,Chilledheart/chromium,axinging/chromium-crosswalk,Just-D/chromium-1,Pluto-tv/chromium-crosswalk,Just-D/chromium-1,Just-D/chromium-1