Add spec for invisible custom property

[xzyfer]

This highlighted a bug in LibSass.

@nex3 do think custom properties be invisible? My feeling is that they should be output regardless.

Note this currently causes an error in Ruby Sass

NoMethodError: undefined method `size' for nil:NilClass
  Use --trace for backtrace.

[skip ruby-sass]
[skip libsass]

[nex3]

--foo:; should be a syntax error. The spec requires a <declaration-value> production, which requires at least one token of some kind. Ruby Sass's behavior is obviously still incorrect.

There's still a question of what to do for --foo:#{""};. We could either drop the property or error out when we detect that the resolved value is empty; my instinct is to do the latter, since it's almost always going to be an error, and I don't want people to end up writing --foo:#{...} instead of --foo: #{...} on purpose to activate the dropping behavior.