Check if p_astro and em_bright need to be downloaded

The code path used to generate initial notices currently always attempts to download p_astro and em_bright files from the superevent, as seen in the orchestrator. I checked on my own machine and found that specifying None as a filename to the gracedb_sdk client returns all of the files (see this comment for an example). This MR calls the identity task instead of the download task if em_bright_filename or p_astro_filename are None. This should fix #687 (closed), but I still need to confirm that on emfollow-dev.