Catch NPE when creating URI

[pulkitsethi]

This PR catches an NPE that is thrown when passing in a null url to MessageCreator.setMediaUrl().

A few reasons it makes sense to do this:

1. MessageCreator.create() itself handles null values for any params:
   

   twilio-java/src/main/java/com/twilio/rest/api/v2010/account/MessageCreator.java

   Line 501 in ce1c451

   if (mediaUrl != null) {

2. Checking for a null mediaUrl string messes up the flow using the API:

    Message message = Message.creator(toPN, fromPN, body)
            .setMediaUrl(mediaUrl)	
            .create();

turns into this:

    MessageCreator messageCreator = Message.creator(toPN, fromPN, body);

    if (!Strings.isNullOrEmpty(mediaUrl)) {
      messageCreator.setMediaUrl(mediaUrl);
    }

    Message message = messageCreator.create();

3. We are already returning null if we throw a URISyntaxException.

twilio-java/src/main/java/com/twilio/converter/Promoter.java

Line 21 in ce1c451

} catch (URISyntaxException e) {

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[childish-sambino]

I'm okay with this since we're already catching URISyntaxException, but I don't like that syntax exceptions are silently swallowed. But that's out-of-scope here.