Function jsonObjectOpt( txt )

Description:
Converts the supplied string to a JSON object which can be interrogated further. If the input doesn't have the syntax of a JSON object, null will be returned.

For most purposes this will behave the same as the jsonObject function, but it may be slower if there are many invalid or empty JSON strings.

Parameters:
txt (String)
JSON object text
Return Value (JSONObject):
JSON object value on which further methods can be called, or null
Signature:
JSONObject jsonObjectOpt(String)