Commit 63205981 removed some events but left all other event IDs at
their original values, which created "holes" at the events IDs array.
The JS backend for mp.register_event maps a name to an ID by scanning
all IDs and stopping when the name was found or a NULL name was
returned. Lua does the same except that it doesn't stop on NULL name.
Previously it was not possible to have a NULL name before the end of
the array, but now it is possible due to the enumeration holes.
Fix by skipping missing names, like lua does.
5c5e35c1bc