Simplify deprecation template logic

As a chart developer I've been inspired by your way of handling deprecations. I find it to be an excellent experience as a consumer of your chart.

So while trying to implement something similar for Mattermost's Helm chart I came up with the simplification in the logic this MR contains.

Instead of comparing the length of strings, it simply looks to see if an array is empty or not.

NOTE: the without function was added to helm 2 years ago, so no worries about introducing it I think.