Predicate methods should return booleans, not strings

This change ensures that the method `complete` actually returns a
boolean, rather than a possibly nil string.

lib/gitlab/diff/diff_refs.rb

@@ -37,7 +37,7 @@ module Gitlab
       # We have `base_sha` directly available on `DiffRefs` because it's faster#
       # than having to look it up in the repo every time.
       def complete?
-        start_sha && head_sha
+        start_sha.present? && head_sha.present?
       end
 
       def compare_in(project)

lib/gitlab/diff/formatters/image_formatter.rb

@@ -23,7 +23,7 @@ module Gitlab
         end
 
         def complete?
-          x && y && width && height
+          [x, y, width, height].all?(&:present?)
         end
 
         def to_h

lib/gitlab/diff/formatters/text_formatter.rb

@@ -19,7 +19,7 @@ module Gitlab
         end
 
         def complete?
-          old_line || new_line
+          old_line.present? || new_line.present?
         end
 
         def to_h