tab: don't interpret unknown deps as no deps

Library/Homebrew/tab.rb

@@ -60,7 +60,6 @@ class Tab < OpenStruct
   def self.from_file_content(content, path)
     attributes = Utils::JSON.load(content)
     attributes["tabfile"] = path
-    attributes["runtime_dependencies"] ||= []
     attributes["source_modified_time"] ||= 0
     attributes["source"] ||= {}
 

Library/Homebrew/test/test_tab.rb

@@ -97,7 +97,7 @@ class TabTests < Homebrew::TestCase
     assert_equal TEST_SHA1, tab.HEAD
     assert_equal :clang, tab.cxxstdlib.compiler
     assert_equal :libcxx, tab.cxxstdlib.type
-    assert_empty tab.runtime_dependencies
+    assert_nil tab.runtime_dependencies
   end
 
   def test_from_file