Fix xcode command crash when OTHER_CFLAGS is not specified

frontend/lib/nullarihyon/xcode.rb

@@ -162,6 +162,8 @@ def run(io)
     end
 
     def self.tokenize_command_line(line)
+      return [] unless line
+
       scanner = StringScanner.new(line)
 
       tokens = []

frontend/test/xcode_test.rb

@@ -138,6 +138,11 @@ class XcodeTest < MiniTest::Spec
       it "returns OTHER_CFLAGS from env" do
         assert_equal %w(-iquote /some/directory -isystem /another/directory), xcode.other_cflags
       end
+
+      it "returns empty array if OTHER_CFLAGS is missing in env" do
+        env.delete("OTHER_CFLAGS")
+        assert_equal [], xcode.other_cflags
+      end
     end
 
     describe "#arc_enabled?" do
@@ -160,6 +165,10 @@ class XcodeTest < MiniTest::Spec
       it "splits quoted command line" do
         assert_equal ["hello world", "good morning"], Xcode.tokenize_command_line("\"hello world\" 'good morning'")
       end
+
+      it "returns empty array when nil is given" do
+        assert_equal [], Xcode.tokenize_command_line(nil)
+      end
     end
   end
 end