JavaBigIntegerParser can throw StringIndexOutOfBoundsException for empty string input

[pjfanning]

Reported to Jackson team FasterXML/jackson-core#1623.
The Javadoc in JavaBigIntegerParser only mentions NullPointerException and NumberFormatException.
So, would it be ok to update the Javadoc to mention StringIndexOutOfBoundsException or alternatively, to add a guard to prevent StringIndexOutOfBoundsException and to ensure NumberFormatException instead?

[wrandelshofer]

Oops. Yeah that is a bug that I need to fix.