tab: default runtime_dependencies to nil.

Library/Homebrew/tab.rb

@@ -185,7 +185,7 @@ class Tab < OpenStruct
       "stdlib" => nil,
       "compiler" => DevelopmentTools.default_compiler,
       "aliases" => [],
-      "runtime_dependencies" => [],
+      "runtime_dependencies" => nil,
       "source" => {
         "path" => nil,
         "tap" => nil,

Library/Homebrew/test/tab_spec.rb

@@ -68,7 +68,7 @@ describe Tab do
     expect(tab.tap).to be nil
     expect(tab.time).to be nil
     expect(tab.HEAD).to be nil
-    expect(tab.runtime_dependencies).to be_empty
+    expect(tab.runtime_dependencies).to be nil
     expect(tab.stable_version).to be nil
     expect(tab.devel_version).to be nil
     expect(tab.head_version).to be nil