autocomplete: Options list does not go away despite having zero options available

[PIG208]

Even if the autocomplete query does not match anything, the options list still appears, though empty.

This is similar to #226, where the options list does not show up until the user starts typing after "@". The underlying cause is probably different——the list does get updated, despite becoming empty, on the keystroke that forms a query with zero results.

"q"	"ql" (no matches)	"qll" (option list goes away)

The issue likely applies to autocompletion in general, because it also happens to the topic input:

[sahu-virendra-1908]

Hi @PIG208 and @gnprice
I came across this issue and would love to work on it. Could you please assign it to me?