Error running batch:map_warnings --with_counts when there are no warnings

[kspurgin]

This batch had errors, not warnings. Out of habit, I ran the wrong command and it was ugly instead of graceful:

❯  thor batch map_warnings --with_counts era
/Users/kristina/code/cs/migration_tools/lib/tasks/batch.thor:250:in `report_warnings_with_counts': undefined method `length' for nil:NilClass (NoMethodError)

      end.last.length
              ^^^^^^^
	from /Users/kristina/code/cs/migration_tools/lib/tasks/batch.thor:142:in `block in map_warnings'
	from /Users/kristina/.rbenv/versions/3.1.0/lib/ruby/gems/3.1.0/gems/dry-monads-1.5.0/lib/dry/monads/result.rb:124:in `either'
	from /Users/kristina/code/cs/migration_tools/lib/tasks/batch.thor:140:in `map_warnings'
	from /Users/kristina/.rbenv/versions/3.1.0/lib/ruby/gems/3.1.0/gems/thor-1.3.0/lib/thor/command.rb:28:in `run'
	from /Users/kristina/.rbenv/versions/3.1.0/lib/ruby/gems/3.1.0/gems/thor-hollaback-0.2.1/lib/thor/hollaback.rb:68:in `run'
	from /Users/kristina/.rbenv/versions/3.1.0/lib/ruby/gems/3.1.0/gems/thor-1.3.0/lib/thor/invocation.rb:127:in `invoke_command'
	from /Users/kristina/.rbenv/versions/3.1.0/lib/ruby/gems/3.1.0/gems/thor-1.3.0/lib/thor.rb:527:in `dispatch'
	from /Users/kristina/.rbenv/versions/3.1.0/lib/ruby/gems/3.1.0/gems/thor-1.3.0/lib/thor/base.rb:584:in `start'
	from /Users/kristina/.rbenv/versions/3.1.0/lib/ruby/gems/3.1.0/gems/thor-1.3.0/lib/thor/runner.rb:41:in `method_missing'
	from /Users/kristina/.rbenv/versions/3.1.0/lib/ruby/gems/3.1.0/gems/thor-1.3.0/lib/thor/command.rb:30:in `run'
	from /Users/kristina/.rbenv/versions/3.1.0/lib/ruby/gems/3.1.0/gems/thor-1.3.0/lib/thor/command.rb:144:in `run'
	from /Users/kristina/.rbenv/versions/3.1.0/lib/ruby/gems/3.1.0/gems/thor-1.3.0/lib/thor/invocation.rb:127:in `invoke_command'
	from /Users/kristina/.rbenv/versions/3.1.0/lib/ruby/gems/3.1.0/gems/thor-1.3.0/lib/thor.rb:527:in `dispatch'
	from /Users/kristina/.rbenv/versions/3.1.0/lib/ruby/gems/3.1.0/gems/thor-1.3.0/lib/thor/base.rb:584:in `start'
	from /Users/kristina/.rbenv/versions/3.1.0/lib/ruby/gems/3.1.0/gems/thor-1.3.0/bin/thor:6:in `<top (required)>'
	from /Users/kristina/.rbenv/versions/3.1.0/bin/thor:25:in `load'
	from /Users/kristina/.rbenv/versions/3.1.0/bin/thor:25:in `<main>'