refactor: add strict nullchecks

[JKRhb]

Another rather trivial PR adding explicit checks for null or undefined.

[coveralls]

Pull Request Test Coverage Report for Build 1314194310

• 44 of 45 (97.78%) changed or added relevant lines in 6 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 90.341%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/agent.js	24	25	96.0%
<!--	Total:	44	45	97.78%	-->

Totals
Change from base Build 1313580033:	-0.1%
Covered Lines:	1078
Relevant Lines:	1163

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💛 - Coveralls

[Apollon77]

honestly I do not like "type-unsafe" checks with == or != ... They are not rteally better then the "falsy" checks before ... (in my eyes) but all good we can leave it that way too ...

[JKRhb]

Yeah, it adds some noise that isn't really necessary :/ The reason for PRs like these were the fact that I activated the option for strict null checks in #293 which complained about checks for falsy values. Maybe we can discuss over there if this is actually necessary. So far I thought the stricter the checks the better but in most cases it probably doesn't bring that much value.

[Apollon77]

I personally use "falsy" checks if I know that it can not be false and want to do tese checks ... or I really check for types or such depending on what I want/need... but as said ... personal flavour