Enable `Lint/EmptyWhen` cop and correct offense

.rubocop.yml

@@ -572,6 +572,10 @@ Lint/ElseLayout:
 Lint/EmptyEnsure:
   Enabled: true
+# Checks for the presence of `when` branches without a body.
+Lint/EmptyWhen:
+  Enabled: true
+
 # Align ends correctly.
 Lint/EndAlignment:
   Enabled: true

.rubocop_todo.yml

@@ -21,10 +21,6 @@ Lint/AmbiguousRegexpLiteral:
 Lint/AssignmentInCondition:
   Enabled: false
-# Offense count: 1
-Lint/EmptyWhen:
-  Enabled: false
-
 # Offense count: 20
 Lint/HandleExceptions:
   Enabled: false

lib/gitlab/diff/parser.rb

@@ -45,7 +45,7 @@ module Gitlab
               line_new += 1
             when "-"
               line_old += 1
-            when "\\"
+            when "\\" # rubocop:disable Lint/EmptyWhen
               # No increment
             else
               line_new += 1