Fixing crash on empty scan

When scanning on one of my phones I ran into this issue when hitting
back without scanning anything.  A quick null check solves the issue,
and it functions normally now.

wallet/src/de/schildbach/wallet/ui/InputParser.java

@@ -53,14 +53,15 @@ public abstract static class StringInputParser extends InputParser
 	{
 		private final String input;
 
-		public StringInputParser(@Nonnull final String input)
+		public StringInputParser(final String input)
 		{
 			this.input = input;
 		}
 
 		@Override
 		public void parse()
 		{
+            if(input == null) return;
 			if (input.startsWith("litecoin:"))
 			{
 				try