Avoid useless isDefinedAndNotNull() check in isEffectCallback() check

Removing this check results in an 8% performance improvement.
__Test:__ test/perf/balls-replace-compositing.html
__Device:__ Nexus 10
__Without patch:__
```
Average: 1.4581406928090335 FPS
Stddev: 0.024775606202678754 FPS
Min: 1.4187670245704818 FPS
Max: 1.5102616996442075 FPS
Median: 1.454523124706587 FPS
```
__With patch:__
```
Average: 1.5757391589373069 FPS
Stddev: 0.013885486397760056 FPS
Min: 1.5620597038094253 FPS
Max: 1.625558592747966 FPS
Median: 1.5697093291425672 FPS
```