Fix onINP() attribution error due to empty entries

[philipwalton]

This PR fixes #451 by removing the logic) that would report 0 INP value with an empty entries array if the code detected that at least one interaction did happen but wasn't reported.

This logic is no longer necessary (given that we observe first-input entries), so as long as we always include first-input entries, there should never be an empty entries array (which is what was causing the TypeError in #451).

Note that code prior this PR assumed that any time a first-input entry was dispatched with a duration value above the durationThreshold of the event observer, then there would always be a correspondoing event entry (and thus the first-input entry was redundant). This turns out to not be true in cases where the first interaction occurs before the DOMContentLoaded event fires, so the fix is to just always include first-input entries (without trying to dedupe). (Note: this may be a Chrome bug that needs addressing, cc: @mmocny.)

/cc: @ErwinHofmanRV