ssl_multicert: fix parsing of lines with missing fields

[ffrank]

Uninitialized is-values on properties seem to result in missing hits during out-of-sync checks [citation needed]. By considering their value to be the empty string, the agent can make correct syncing decisions now.

Note that puppet resource trafficserver_ssl_multicert still works as expected and will not include bogus empty string values in the manifest output.

[ffrank]

Fix for issue #18

[igalic]

we may also need this same approach in the other provider(s) that use(s) parsedfile.

[igalic]

then again, given the rather simple structure of trafficserver_plugin, i may be wrong there.