Make new version of JNU_GetStringPlatformChars which checks for null characters

JNU_GetStringPlatformChars needs to check for null characters which are allowed in Java Strings but not in native strings. IllegalArgumentException should be thrown as a fallback. Probably, strings should be checked before calling this function so more specific action should be taken.

A new native function JNU_GetStringPlatformCharsStrict will be added which will be documented to throw IllegalArgumentException
if the returned string contains null characters. Separate PRs will be required to change existing code to use this new function.