Handle empty topics and partitions

[therve]

Using None as a default value for everything doesn't work with Agent
6. We make it work by instead passing empty dict and list when we want
all elements.

[codecov]

Codecov Report

Merging #3807 into master will decrease coverage by 5.65%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3807      +/-   ##
==========================================
- Coverage   86.57%   80.92%   -5.66%     
==========================================
  Files         730        8     -722     
  Lines       37993      540   -37453     
  Branches     4542      113    -4429     
==========================================
- Hits        32893      437   -32456     
+ Misses       3861       72    -3789     
+ Partials     1239       31    -1208

[hithwen]

What happens with people who already have empty vaues on their cfg? Should this be a major?

[dabcoder]

What happens with people who already have empty vaues on their cfg? Should this be a major?

Currently testing this, existing configurations with empty values won't return any metrics, without any errors.