pex: empty headline

### Subject of the issue
Empty heading leads to an error message.
>3#
l

### Your environment
pex.pl, v2.13/20.04.2016 (Perl 5.022001)

### Steps to reproduce
Enter
>3#
Invoker make

### Expected behaviour
Empty heading should be ignored.

### Actual behaviour
Meaningless internal error is reported.
