fix: correct EventEmitter generic type across lib

[jelbourn]

R: @tinayuangao
CC: @kara

The default generic type of EventEmitter is {}. If TS has strictNullChecks enabled, emitting null on such an emitter will cause a build error:

Type 'EventEmitter<{}>' is not assignable to type 'EventEmitter<null>'

[tinayuangao]

LGTM

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