Add back empty cache detection

As part of the refactors the empty stestr timing cache removal code was
removed from the travis config, but the cache wasn't removed. Adding the
python 3.8 job causes a failure because travis has no cached timing data
but still creates the empty directory which triggers
mtreinish/stestr#266. This adds back the empty cache removal to
workaround it so we can use the timing data for scheduling in future
runs.

.travis.yml

@@ -33,6 +33,12 @@ stage_generic: &stage_generic
     - stestr run
   after_failure:
     - python tools/report_ci_failure.py
+  before_script:
+    - |
+      if [ ! "$(ls -A .stestr)" ]; then
+          rm -rf .stestr
+      fi
+
 
 stage_linux: &stage_linux
   <<: *stage_generic