Gracefully handle empty custom properties

[nex3]

Empty custom properties like --foo:; should produce syntax errors, but currently Ruby Sass throws:

/home/nweiz/code/sass/lib/sass/util.rb:159:in `merge_adjacent_strings': undefined method `size' for nil:NilClass (NoMethodError)
        from /home/nweiz/code/sass/lib/sass/tree/prop_node.rb:73:in `initialize'
        from /home/nweiz/code/sass/lib/sass/scss/parser.rb:837:in `new'
        from /home/nweiz/code/sass/lib/sass/scss/parser.rb:837:in `css_variable_declaration'
        from /home/nweiz/code/sass/lib/sass/scss/parser.rb:785:in `try_declaration'
        from /home/nweiz/code/sass/lib/sass/scss/parser.rb:732:in `declaration_or_ruleset'
        from /home/nweiz/code/sass/lib/sass/scss/parser.rb:697:in `block_child'
        from /home/nweiz/code/sass/lib/sass/scss/parser.rb:686:in `block_contents'
        from /home/nweiz/code/sass/lib/sass/scss/parser.rb:678:in `block'
        from /home/nweiz/code/sass/lib/sass/scss/parser.rb:670:in `ruleset'
        from /home/nweiz/code/sass/lib/sass/scss/parser.rb:696:in `block_child'
        from /home/nweiz/code/sass/lib/sass/scss/parser.rb:686:in `block_contents'
        from /home/nweiz/code/sass/lib/sass/scss/parser.rb:137:in `stylesheet'
        from /home/nweiz/code/sass/lib/sass/scss/parser.rb:41:in `parse'
        from /home/nweiz/code/sass/lib/sass/engine.rb:414:in `_to_tree'
        from /home/nweiz/code/sass/lib/sass/engine.rb:290:in `render'
        from /home/nweiz/code/sass/lib/sass/exec/sass_scss.rb:400:in `run'
        from /home/nweiz/code/sass/lib/sass/exec/sass_scss.rb:63:in `process_result'
        from /home/nweiz/code/sass/lib/sass/exec/base.rb:52:in `parse'
        from /home/nweiz/code/sass/lib/sass/exec/base.rb:19:in `parse!'
        from /home/nweiz/code/sass/bin/sass:13:in `<main>'

Also, empty interpolated custom properties like --foo:#{""} should throw an error but Ruby Sass currently emits them as-is.

See sass/sass-spec#1249 and sass/sass-spec#1259