SoftButtonObject doesn't handle error states correctly

[joeljfischer]

Bug Report

The SoftButtonObject can be created with states that don't exist or are incorrect. We should check these states on construction to ensure that developers can't send incorrect soft buttons.

1. Creating a SoftButtonObject with empty states should throw an exception
2. Creating a SoftButtonObject with states where the initial state name is not among the set states.

Reproduction Steps

1. Create a soft button object with empty state list
2. Send the soft button object

Expected Behavior

The soft buttons should throw an exception

Observed Behavior

The soft buttons return a success because no soft buttons are sent in the Show

OS & Version Information

• Android Version: n/a
• SDL Android Version: v5.3.1
• Testing Against: Sync 3

Test Case, Sample Code, and / or Example App

n/a