Fix regexp replace to return empty string when pattern does not match

[iandees]

Root cause

In row_fxn_regexp() (openaddr/conform.py), the replace branch used re.sub() directly on the field value. re.sub() returns the original, unmodified string when the pattern never matches anywhere in it, so a row whose field didn't match the configured pattern silently kept the full original field value instead of becoming empty.

This is inconsistent with the non-replace branch, which explicitly falls back to '' when there's no match (if match else '').

Reported in #57: a regexp conform on REV_LongLabel was expected to produce an empty number for rows where the field held only a zip/city/state/country (no house number to match), but it returned the full field value instead.

Fix

Check pattern.search(value) before calling re.sub(); when there is no match, set the field to '', matching the established no-match convention from the non-replace branch. When there is a match, behavior is unchanged (re.sub still runs as before).

Verification

• Added a regression test (regex split - replace - bad match) alongside the existing test_row_fxn_regexp test, covering replace-mode with a pattern that does not match the input field, asserting the field becomes ''.
• Confirmed the new test fails on the pre-fix code (oa:street came back as the full original field '123 MAPLE ST' instead of ''), then confirmed it passes after the fix.
• Ran the full conform test suite in the project's docker test image: python3 -m unittest openaddr.tests.conform -v — all 65 tests pass, including existing replace-mode tests that do match (no regression there).

Fixes #57