Fix: SendImpressionEvent will return false when event is not sent

[mnoman09]

Summary

• SendImpressionEvent will return false when event is not sent and return true when sent successfully
• In case of userEvent is null it will return false

[coveralls]

Pull Request Test Coverage Report for Build 1654

• 2 of 3 (66.67%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.002%) to 89.956%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core-api/src/main/java/com/optimizely/ab/Optimizely.java	2	3	66.67%

Totals
Change from base Build 1648:	-0.002%
Covered Lines:	4451
Relevant Lines:	4948

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 1656

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 89.976%

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Totals
Change from base Build 1648:	0.02%
Covered Lines:	4452
Relevant Lines:	4948

---

💛 - Coveralls

[jaeopt]

LGTM - I added more tests for decisionEventDispatched

[msohailhussain]

please update Copyright header.

[msohailhussain]

lgtm