Fix PHP 8.1 "Passing null to parameter" deprecations in regex_replace modifier

[h-marumoto]

Problem

The regex_replace modifier triggers Passing null to parameter deprecation errors on PHP >= 8.1 when any of its arguments is null, e.g. when a template variable holds null or was never assigned:

{$nullableVar|regex_replace:"/[\r\t\n]/":" "}

Deprecated: preg_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated

The same applies to:

• a null search pattern (strpos() and preg_match() inside regex_replace_check(), and preg_replace()'s $pattern)
• a null replacement (preg_replace()'s $replacement)
• a null limit (preg_replace()'s $limit)

Fix

Follow the existing convention already used in DefaultExtension for number_format, explode, split and implode (see the "provide $string default to prevent deprecation errors in PHP >=8.1" comments): coalesce null arguments before passing them on.
Note on backwards compatibility: this is strictly a deprecation fix and does
not change any output.

• $string ?? '' and $replace ?? '' produce exactly what PHP's implicit null-to-string coercion produced before.
• $limit ?? 0 deliberately falls back to 0 (not the parameter default -1), because PHP's null-to-int coercion has always turned a null limit into 0 (i.e. no replacements). Falling back to -1 would silently change the output of existing templates that pass a null limit. A comment in the code documents this.

Tests

Added a test covering all three null cases. Since the test suite mutes E_DEPRECATED globally (PHPUnit_Smarty::setUp()), the test also collects deprecations via a temporary error handler and asserts that none are raised — running it without the fix fails with the three deprecations listed above, and with the fix all existing PluginModifier tests still pass unchanged.

This is the same kind of fix as #834 / #838 / #1135.