#7205 fix dataset page load

[sekmiller]

What this PR does / why we need it:
The Dataset Page fails to load when a tabular file has an original file size of null. A one line change assigns a value of zero to the size when it is null so that the page will load properly

Which issue(s) this PR closes:

Closes #7205

Special notes for your reviewer:
This also suppresses the download original at the dataset level if the full size of the original files is zero (none have an original file size.)

Suggestions on how to test this:
Null out the original file size of a tabular file and verify that its dataset page will load.
update datatable set originalfilesize = null where datafile_id = ??;

Does this PR introduce a user interface change? If mockups are available, please link/include them here:
no
Is there a release notes update needed for this change?:
Not really unless we want to remind installations to run the fixmissingoriginalsizes api

Additional documentation:

[pdurbin]

I didn't test the fix myself but it's small and simple and makes sense.