guard for nil suffix in String#has_suffix?()

lib/rscons/monkey/string.rb

@@ -1,7 +1,9 @@
 class String
   def has_suffix?(suffix)
-    suffix = [suffix] if suffix.is_a?(String)
-    suffix.find {|s| self =~ /#{s}$/}
+    if suffix
+      suffix = [suffix] if suffix.is_a?(String)
+      suffix.find {|s| self =~ /#{s}$/}
+    end
   end
 
   def set_suffix(suffix = '')