ext/tidy: Accept null for the $encoding of the repair functions

[lacatoire]

tidy_repair_string(), tidy_repair_file(), tidy::repairString() and tidy::repairFile() declare ?string $encoding = null but parse it with Z_PARAM_STRING, so passing null is rejected. All other tidy entry points already use Z_PARAM_STRING_OR_NULL for this parameter (628db3f). null is then handled like an omitted argument: enc_len is 0 and the encoding is not set.

Before:

$ php -r 'declare(strict_types=1); tidy_repair_string("<p>test", null, null);'
Fatal error: Uncaught TypeError: tidy_repair_string(): Argument #3 ($encoding) must be of type string, null given

After:

$ sapi/cli/php -r 'declare(strict_types=1); var_dump(strlen(tidy_repair_string("<p>test", null, null)) > 0);'
bool(true)

$ sapi/cli/php run-tests.php ext/tidy
Tests passed    :    51 (98.1%)
Tests failed    :     0 (  0.0%)

The new test tidy_repair_encoding_null.phpt fails without the fix (the TypeError above) and passes with it.