nuageinit: Add guards against empty user data

Add guards against attempting to process a user data file with an empty
first line or contents.

PR: 290395
Reviewed by: bapt (earlier), dtxdf, markj
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D53239

(cherry picked from commit 57d25b6333523491ac7a3e869bd5d16127203eed)