File select cancellation fix + condition cleanup

[jarmoza]

This bug fix addresses issue #194. Before, selecting a data table file (participants.tsv) and then attempting to select a second file but instead cancelling in the file select dialog resulted in a JS error and page crash.

Once this crash was initially addressed, it also revealed that the code would wipe the first data table selection (as well as its filename and file preview in the UI).

Neither of these issues occurred for the data dictionary file selection.

Expected behavior:

• Page does not crash when second data table file selection attempt is cancelled
• Currently selected data table is not wiped but remains after file select cancellation
• Data dictionary file selection maintains consistent behavior with data table file selection

The above was addressed by creating a cleaner check against file select event's file data and by not resetting the file data variable if no new file has been selected. (See onFileSelected in file-selector.vue.)

I also cleaned up some conditional checks for possible values of the file data variable fileInput in file-selector. It's either null or has a valid value. And I removed the file-select event propagating back up to index.vue that was using the string "none" to indicate a file selection cancellation.

[surchs]

Thanks for the PR @jarmoza. This seems to be a MacOS specific bug, as I couldn't reproduce on Linux or Windows. Very interesting.

The changes look good and make sense to me. I'd merge this!