Fix expect_offense when annotations missing

Fix crash in expect_offense where multiple offenses are detected by the
cop by some were not included in the annotated source. For example:

```ruby
expect_offense(<<~RUBY)
  x % 2 == 0
  ^^^^^^^^^^ Replace with `Integer#even?`
  x % 2 != 0
  # second offense missing
RUBY
```

lib/rubocop/rspec/expect_offense.rb

@@ -235,7 +235,7 @@ def ==(other)
         def match_annotations?(other)
           annotations.zip(other.annotations) do |(_actual_line, actual_annotation),
                                                  (_expected_line, expected_annotation)|
-            if expected_annotation.end_with?(ABBREV)
+            if expected_annotation&.end_with?(ABBREV)
               if actual_annotation.start_with?(expected_annotation[0...-ABBREV.length])
                 expected_annotation.replace(actual_annotation)
               end