shims/mac/super/xcrun: unset rather than emptying DEVELOPER_DIR

Library/Homebrew/shims/mac/super/xcrun

@@ -4,14 +4,14 @@
 # it and attempts to avoid these issues.
 
 # These could be used in conjunction with `--sdk` which ignores SDKROOT.
-if [[ "$*" =~ (^| )-?-show-sdk-(path|version|build) ]]; then
+if [[ "$*" =~ (^| )-?-show-sdk-(path|version|build) && -n "$HOMEBREW_DEVELOPER_DIR" ]]; then
   export DEVELOPER_DIR=$HOMEBREW_DEVELOPER_DIR
 else
   # Some build tools set DEVELOPER_DIR, so discard it
   unset DEVELOPER_DIR
 fi
 
-if [ -z "$SDKROOT" ]; then
+if [[ -z "$SDKROOT" && -n "$HOMEBREW_SDKROOT" ]]; then
   export SDKROOT=$HOMEBREW_SDKROOT
 fi