RegexReplacement static factory method doesn't handle null 'replacement' attribute correctly

RegexReplacement#createRegexReplacement

Static builder method returns `null`if the `regex`attribute is null, but although it logs an error it does not return `null` if the `replacement` attribute is `null`.

```
public static RegexReplacement createRegexReplacement(
@PluginAttribute("regex") final Pattern regex, @PluginAttribute("replacement") final String replacement) {
if (regex == null) {
LOGGER.error("A regular expression is required for replacement");
return null;
}
if (replacement == null) {
LOGGER.error("A replacement string is required to perform replacement");
}
// FIXME: should we use Matcher.quoteReplacement() here?
return new RegexReplacement(regex, replacement);
}
```