Avoid UB trimming empty response header values

[mcc0nnell]

This avoids undefined behavior when trimming trailing whitespace from empty header values.

Several response/header parsing paths initialized the trim pointer with &value[strlen(value) - 1] (or the equivalent l + strlen(l) - 1). For an empty value, that constructs a pointer before the start of the string object before the loop condition can reject it.

Use an end-exclusive pointer instead and only inspect end[-1] while end > value.

Updated paths:

• mod_proxy_http
• mod_proxy_uwsgi
• mod_proxy_hcheck
• server/util_script.c
• mod_cern_meta

The proxy tests add backend responses containing a valid empty X-Empty: header for both HTTP proxy and uWSGI paths.

Validation against trunk 3ffb2beeb058fa88d0a7b0de8b93d21ac7ec5f66:

• built and installed Apache/2.5.1-dev from this patched tree in a user-space prefix
• all touched C paths compiled successfully
• focused new regressions: 2/2 passed
• complete test_03_response.py + test_05_uwsgi.py: 6/6 passed
• git diff --check: clean

A small Clang UBSan/pointer-overflow reproducer also reports the original empty-value expression and is silent with the end-exclusive form.

I have not demonstrated a crash, out-of-bounds read/write, information disclosure, or other exploit primitive; this is a narrow UB hardening change.