Don't print out an empty tags list for postgres

[alq666]

This, coupled with incorrect tag handling in the agent, caused the agent to crash whenever the postgres instance was used without tags.

If this is good to go, it'll be 1.0.2.

[miketheman]

1. Should we do this for every integration that can have optional tags?
2. Please leave off the version-specific changes in the second commit - I have to commit a Changelog WIP I have for this, and metadata gets updated on release via knife-community

[alq666]

I grepped the code and it was the only instance I found that had a missing if statement.

[alq666]

Please merge at your convenience if this works for you.