[BUG] `String.first` and `String.last` raise `IndexOutOfRangeException` when applied to empty strings

[omaus]

Description

String.first and String.last return IndexOutOfRangeException when applied to empty strings. It would be better to instead check for empty strings and throw an explicit error message.

Repro steps

Please provide the steps required to reproduce the problem

1. Write

   String.first ""
   String.last ""

2. Execute

Expected behavior

Proper error message.

Actual behavior

System.IndexOutOfRangeException.