m7-common: fix breakfast sync error if no vendor

Let breakfast retrieve repositories without error if vendor is not yet
populated (which for most users it should not be, as the extract scripts
have not yet been run).

Change-Id: I34650c8c41b9bb6ff501d60fd9ec91b546297d8f
Merge remote-tracking branch 'upstream/cm-11.0' into cm-11.0