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
f49a4440d3