DEV: support nil values in the eq_time matcher (#22116)

We use it like this:

    expect(message.created_at).to eq_time(created_at)

The problem is that if one of the values or both of them are `nil` the matcher fails 
with this error:

    NoMethodError: undefined method `-' for nil:NilClass

This commit adds support for `nil` values. If both time values are `nil` they are equal, 
if only one value is `nil` they aren't.

spec/support/time_matcher.rb

@@ -11,7 +11,11 @@ RSpec::Matchers.define :be_within_one_minute_of do |expected_time|
 end
 
 RSpec::Matchers.define :eq_time do |expected_time|
-  match { |actual_time| (actual_time - expected_time).abs < 0.001 }
+  match do |actual_time|
+    return true if expected_time.nil? && actual_time.nil?
+    return false if expected_time.nil? || actual_time.nil?
+    (actual_time - expected_time).abs < 0.001
+  end
   failure_message { |actual_time| "#{actual_time} is not within 1 millisecond of #{expected_time}" }
   failure_message_when_negated do |actual_time|
     "#{actual_time} is within 1 millisecond of #{expected_time}"