Fix rtl string qa NULL-deref crashes (test bug + source hardening)

[leginee]

This PR is the first out of 3 fixes from test migration. The original test did not make sense to me. I added the test for NULL pointers in the code itself as guards. And turned the tests into boundry tests.
The code is ai generated. best is to have more eyes to check it.

commit message for conveniance

Error case 1 of the test migration: ~40 *_000 cases in the rtl string qa suites passed NULL into C string functions that dereference it (e.g. rtl_str_compare(NULL, NULL)), causing 0xC0000005 AVs. These were dormant under the old dmake build and only surface now that the tests actually run. NULL violates the functions' documented contract ("must be null-terminated"), so the defect was in the tests, not the (correct, unchanged) source. Fixed both sides for defense in depth.

Tests (qa/rtl/ostring/rtl_str.cxx, rtl_string.cxx, qa/rtl/oustring/rtl_ustr.cxx):

• Rewrote the UB NULL-deref cases as contract-respecting boundary tests (empty string, prefix/ordering-sign < 0 / > 0), which also closes a previously-untested coverage gap (result sign was never asserted).
• Added real assertions to the safe NULL-with-length-0 cases, which document the length-bounded functions' tolerance as a regression guard.

Source (rtl/source/strtmpl.c, ustring.c):

• Added entry-point NULL guards: OSL_PRECOND (diagnoses misuse loudly in non-product builds, compiles out in product builds) plus a defined empty-string fallback so the library never dereferences NULL. Guards sit at function entry, outside the per-character loops, so string-processing throughput is unchanged.
• strtmpl.c: one edit covers both the sal_Char and sal_Unicode instantiations. getLength is the choke point (guarding it transitively protects hashCode, lastIndexOf*, indexOfStr, trim); compare/compareIgnoreAsciiCase/indexOfChar/replaceChar/ toAscii{Lower,Upper}Case/valueOfChar guarded directly.
• ustring.c: guarded the 6 mixed UTF-16/ASCII comparison helpers; length-bounded args clamp the length to 0 to avoid NULL+0 pointer arithmetic.

BUILD.bazel: rtl_str/rtl_ustr/rtl_string removed from the "known upstream failures" notes; they now pass.

Verified: sal3.dll rebuilds (both template instantiations) and rtl_str / rtl_ustr / rtl_string all pass.

[DamjanJovanovic]

Please remove these NULL-pointer guards in release mode for better performance, and only use them in debug builds.

[leginee]

do you think their performance impact is that visible?

[leginee]

@DamjanJovanovic : switches added.