Merge pull request #8853 from jonchang/fix-nil-mtime

tab: fix nil modification time

.github/workflows/doctor.yml

 name: brew doctor
 on:
-  push:
+  pull_request:
     paths:
       - .github/workflows/doctor.yml
       - Library/Homebrew/cmd/doctor.rb

Library/Homebrew/tab.rb

@@ -324,7 +324,7 @@ class Tab < OpenStruct
   end
 
   def source_modified_time
-    Time.at(super)
+    Time.at(super || 0)
   end
 
   def to_json(options = nil)