scripts/oe-build-perf-report: fix dumping buildstats

Fix a misbehavior when some of the buildstats are missing.

(From OE-Core rev: 2930515acb5cfdcd335a76ae36fd9f7189207aa3)

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/oe-build-perf-report

@@ -422,6 +422,7 @@ def dump_buildstats(repo, outdir, notes_ref, revs):
                                                   'show', tag + '^0']))
             except GitError:
                 log.warning("Buildstats not found for %s", tag)
+                bs_all = {}
                 missing = True
             for measurement, buildstats in bs_all.items():
                 tag_base, run_id = tag.rsplit('/', 1)