Don't print module header if nothing is defined

Modules may decide not to output any build definitions in some cases.
Clean up the ninja file by not adding headers for empty sections.

One particular usecase is my upcoming multi-stage bootstrapping -
bootstrap_go_* will not output any rules in the first stage unless it's
required to build the primary builder.

Change-Id: I6a6b54da7e1702c63bfa736bcf8daf16956f9449