[gdb/testsuite] Fix gdb.python/py-startup-opt.exp with empty PYTHONDONTWRITEBYTECODE

[gdb/testsuite] Fix gdb.python/py-startup-opt.exp with empty PYTHONDONTWRITEBYTECODE

When running test-case gdb.python/py-startup-opt.exp with empty
PYTHONDONTWRITEBYTECODE:
...
$ cd build/gdb/testsuite
$ PYTHONDONTWRITEBYTECODE= make check \
    RUNTESTFLAGS=gdb.python/py-startup-opt.exp
...
I get:
...
end^M
dont_write_bytecode is off^M
(gdb) FAIL: $exp: attr=dont_write_bytecode: testname: input 6: end
...

The problem is that the test-case expects dont_write_bytecode to be
on, which is incorrect because PYTHONDONTWRITEBYTECODE only has effect if set
to a non-empty string [1].

Fix this by correctly setting expectations in the test-case.

Tested on x86_64-linux, with:
- PYTHONDONTWRITEBYTECODE=
- PYTHONDONTWRITEBYTECODE=1
- unset PYTHONDONTWRITEBYTECODE

Approved-By: Tom Tromey <tom@tromey.com>
[1] https://docs.python.org/3/using/cmdline.html#envvar-PYTHONDONTWRITEBYTECODE