Only call File.file? when path present

Calling `iseq.absolute_path` might return nil.

Author: adam12

lib/debug/source_repository.rb

@@ -16,7 +16,7 @@ def add iseq, src
     def add_path path, src: nil
       case
       when src
-        if File.file?(path)
+        if path && File.file?(path)
           path = '(eval)' + path
           src = nil
         end