Meta: Prevent "HOMEBREW_PREFIX: unbound variable" in shell_include.sh

...when this is used in scripts that `set -u`.

No intended behavior change.

Author: nico

Meta/shell_include.sh

@@ -37,7 +37,7 @@ find_executable() {
   paths=("/usr/sbin" "/sbin")
 
   if [ "$(uname -s)" = "Darwin" ]; then
-    if [ -n "${HOMEBREW_PREFIX}" ]; then
+    if [ -n "${HOMEBREW_PREFIX:-}" ]; then
       paths+=("${HOMEBREW_PREFIX}/opt/e2fsprogs/bin" "${HOMEBREW_PREFIX}/opt/e2fsprogs/sbin")
     elif command -v brew > /dev/null 2>&1; then
       if prefix=$(brew --prefix e2fsprogs 2>/dev/null); then