Make "else" clause optional in if/else statements (#2002)

[nfogh]

Essentially does what you suggested in #2002

[casey]

I'm hesitant to merge at the moment. The empty string is the current preferred "false" value, but long-term, it's not what we want. Long term, I want to make all variables in just lists of strings, and use the empty list as the false value. This may wind up being a breaking change for places which can produce false values, so I'm hesitant to add more places which can produce false values.