podcast: podcast download should not fail when local file is empty

crvirequested to merge
crvi/rhythmbox:podcast-empty-file-issue into master

This MR fixes 3 issues.

  1. Allow podcast download to be initiated even if local_size = 0 and data->download_size = 0, in case if we are not able to ascertain download_size value from GIO. We allow the download to proceed.
  2. Prevent podcast download from failing with File exists error, in case of empty local file, due to g_file_create() usage.
  3. Do not print "podcast partly downloaded (0 of 1231231)" in debug message.

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