Merge pull request #879 from mtsmfm/consider-falsy-in-compute-if-absent

Consider falsy value on `Concurrent::Map#compute_if_absent` fast non-blocking path

lib/concurrent-ruby/concurrent/collection/map/mri_map_backend.rb

@@ -19,7 +19,7 @@ def []=(key, value)
       end
 
       def compute_if_absent(key)
-        if stored_value = _get(key) # fast non-blocking path for the most likely case
+        if NULL != (stored_value = @backend.fetch(key, NULL)) # fast non-blocking path for the most likely case
           stored_value
         else
           @write_lock.synchronize { super }