f49a4440d3
An empty history entry can happen by entering the expression evaluator an immediately hitting enter: ``` $ lldb (lldb) e Enter expressions, then terminate with an empty line to evaluate: 1: <hit enter> ``` The next time the user enters the expression evaluator, if they hit the up arrow to load the previous expression, lldb crashes. This patch treats empty history sessions as a single expression of zero length, instead of an empty list of expressions. Fixes http://llvm.org/PR49845. Differential Revision: https://reviews.llvm.org/D100048