Fix build failure when no Overview.html files exist (#1166)

Add shopt -s nullglob so the Overview.html glob expands to nothing
instead of the literal pattern when there are no matches, which was
causing cp to fail.

Author: sideshowbarker

.github/workflows/build-specs.yml

@@ -39,6 +39,7 @@ jobs:
       - name: Build specs
         run: |
           set -e
+          shopt -s nullglob
           # Handle non-bikeshed specs.
           for file in ./**/Overview.html; do
             cp "$file" "$(dirname "$file")/index.html"