Fixes syntax error in older versions of graphviz.

Older versions of graphviz (tested on version 2.26.3 (20100126.1600))
complains when an attribute is empty without quotes. [label=] fails but
[label=""] works, so the quote_if_necessary function has been updated to
accommodate this.
Merge pull request #8 from gustavla/master

Fixes syntax error in older versions of graphviz.