prevent file=nil panic if syntax error in Blueprints

Bug: 65683273
Test: build/soong/scripts/diff_build_graphs.sh \
      'build/blueprint:work^^^' 'build/blueprint:work'
Test: put a syntax error in a file and see that the
      reported error reports the location of the violation

Change-Id: Iaeedb91ea8e816cb8e9ee954f21cd6c6bc4afa48