we should never return a reference to local variable.
so in this change, a reference to a static variable is returned
instead. this should address following warning from Clang 17:
```
/home/kefu/dev/scylladb/tools/schema_loader.cc:146:16: error: returning reference to local temporary object [-Werror,-Wreturn-stack-address]
return {};
^~
```
Fixes#12875
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closes#12876
6eab8720c4