-update_golden creating empty files (#277)
-update_golden creating empty files (#277)

* confgenerator: -update_golden creating empty files

`updateOrCompareGolden` did not detect the case where a diff is empty
because the file doesn't exist yet. Added a check for the edge case that
creates the empty golden file if it doesn't exist yet.

* confgenerator: reword comment

The other comments don't use self-referring words like "we".

* confgenerator: combine file-writing logic branches

* confgenerator: handle ErrNotExist specifically

The previous implementation assumed any error occurs because the file
does not exist. Instead, check that the returned error is specifically
os.ErrNotExist.

* confgenerator: always write file if empty

To simplify the logic when detecting a golden update, treat an empty
`actual` as something that always needs to be written.

* confgenerator: clarify update golden condition

Added clarifying to the condition that decides whether to (over)write
the golden file.
GoogleCloudPlatform/ops-agent,GoogleCloudPlatform/ops-agent