fix(IDX): allow empty OVERRIDE_DIDC_CHECK

This fixes bash errors when OVERRIDE_DIDC_CHECK is unset, by defaulting
it to the empty string. This is standard best practice with bash
variable and fixes test failures in some environments.

bazel/candid.bzl

@@ -8,7 +8,7 @@ def _did_git_test_impl(ctx):
 
 set -xeuo pipefail
 
-if [[ $OVERRIDE_DIDC_CHECK == "true" ]]; then
+if [[ ${{OVERRIDE_DIDC_CHECK:-}} == "true" ]]; then
     echo "Override didc check requested. Skipping didc_check."
     exit 0
 fi