feat(minecraft): allow values to fall back to server.properties default

[itzg]

This solves these things:

• simplifies the ugly, repetitive two lines of env name-values
• provides a common mechanism to allow for null'ing out a value, such as --set minecraftServer.maxWorldSize=, leaving out the env var, and allowing server.properties value to be used instead.
• gets rid of inconsistent/redundant use of default "" clauses

@billimek , if this looks good to you, then I'll do the same for the other three charts.

For #143

[itzg]

That's ok, I had forgotten I put this change out there 😀