Fix printing of global variable stubs if no inferior is running

Fix printing of global variable stubs if no inferior is running

Since 3c45e9f915ae4aeab7312d6fc55a947859057572 gdb crashes when trying
to print a global variable stub without a running inferior, because of
a missing nullptr-check (the block_scope function took care of that
check before it was converted to a method).

With this check it works again:
```
(gdb) print s
$1 = <incomplete type>
```

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31128
Approved-By: Tom Tromey <tom@tromey.com>
(cherry picked from commit 576745e26c0ec76a53ba45b20af464628a50b3e4)