Avoid ZeroCollators error in solochains

[tmpolaczyk]

In dancebox, the pallet_collator_assignment has a check to ensure that we never return an assignment with 0 collators, because that would brick the chain. But in dancelight, that's not a problem because validators are stored in another pallet. So we can safely handle the case of 0 collators, by returning an empty assignment.

So we will no longer see this log when starting a dancelight node:

2024-10-01 17:03:07 Error in collator assignment, will keep previous assignment. ZeroCollators    

Also add tests to ensure this doesn't panic.

[girazoki]

thank you @tmpolaczyk looks good to me

[github-actions]

Coverage Report

(master)

@@                         Coverage Diff                         @@
##           master   tomasz-zero-collators-solochain      +/-   ##
===================================================================
+ Coverage   66.66%                            66.70%   +0.04%     
  Files         297                               297              
+ Lines       51830                             51893      +63     
===================================================================
+ Hits        34551                             34613      +62     
+ Misses      17279                             17280       +1     

Files Changed	Coverage
/solo-chains/runtime/dancelight/src/lib.rs	66.67% (-0.06%)	🔽

Coverage generated Wed Oct 2 11:20:18 UTC 2024