Revert "Map also empty dictionaries to file"

This reverts commit f54428f (nedbat#1347),
which was a big regression in performance and file sizes when using
dynamic_context, and is no longer needed for nedbat#972 as of
4cc3292 (nedbat#1538).

Fixes nedbat#1586.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

coverage/collector.py

@@ -456,7 +456,7 @@ def mapped_file_dict(self, d: Mapping[str, T]) -> Dict[str, T]:
             assert isinstance(runtime_err, Exception)
             raise runtime_err
 
-        return {self.cached_mapped_file(k): v for k, v in items}
+        return {self.cached_mapped_file(k): v for k, v in items if v}
 
     def plugin_was_disabled(self, plugin: CoveragePlugin) -> None:
         """Record that `plugin` was disabled during the run."""