sh: Handle empty hostname and $PWD when building prompt

[vangyzen]

Summary

If the hostname is empty and \h is used in $PS1,
the remainder of the prompt following \h will be empty.
Likewise for $PWD and \w.

Test Plan

I set PS1='\h \W \$ ' and tested with an empty
hostname and $PS1.

[vangyzen]

Oops. Only the sh/parser.c change should be here. Ignore the others.

[jilles]

OK for the bin/sh/parser.c change.