Still return string if error raised formatting code

If exceptions occured while formatting code,
OutputProcessor#format would return `nil`. Now it should return
a string instead so it can be used safely.

lib/brakeman/processors/output_processor.rb

@@ -10,7 +10,7 @@ class Brakeman::OutputProcessor < Ruby2Ruby
 
   #Copies +exp+ and then formats it.
   def format exp
-    process exp.deep_clone
+    process(exp.deep_clone) || ""
   end
 
   alias process_safely format