autogazelle: use default syntax for expanding $1 (#627)

When calling bazel without parameters, the script was giving an error "$1: unbound variable".
This fixes it by using an explicit default value of an empty string.

cmd/autogazelle/autogazelle.bash

@@ -23,7 +23,7 @@
 
 set -euo pipefail
 
-case "$1" in
+case "${1:-}" in
   build|coverage|cquery|fetch|mobile-install|print_action|query|run|test)
     "$BAZEL_REAL" run @bazel_gazelle//cmd/autogazelle -- -gazelle=//:gazelle
     echo "done running autogazelle" 1>&2