[#221] robustness against nil file lists (such as public_header_files)

lib/cocoapods/command/spec.rb

@@ -302,6 +302,7 @@ module Pod
             patterns = spec.send(accessor.to_sym)
             # Some values are multiplaform
             patterns = patterns.is_a?(Hash) ? patterns.values.flatten(1) : patterns
+            patterns = patterns.compact # some patterns may be nil (public_header_files, for instance)
             patterns.each do |pattern|
               # Skip Filelist that would otherwise be resolved from the working directory resulting
               # in a potentially very expensi operation

lib/cocoapods/local_pod.rb

@@ -139,6 +139,7 @@ module Pod
 
     def expanded_paths(platforms_with_patterns, options = {})
       patterns = platforms_with_patterns.is_a?(Hash) ? platforms_with_patterns[@platform.name] : platforms_with_patterns
+      return [] if patterns.nil?  # some patterns may be nil (specification.public_header_files, for instance)
       patterns.map do |pattern|
         pattern = root + pattern