fix: discovery missing targetValue when runs first time

[varet80]

During initial discovery, while the setTopic value is undefined there is
an issue which arises with an nill value for command_topic on hass
discovery. This issue comes due to the "split" node compared to previous
driver, which combined both.

Check if node is ready before discovering

[coveralls]

Pull Request Test Coverage Report for Build 406059615

• 0 of 2 (0.0%) changed or added relevant lines in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.1%) to 27.935%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/Gateway.js	0	2	0.0%

Files with Coverage Reduction	New Missed Lines	%
lib/Gateway.js	2	20.5%

Totals
Change from base Build 406057957:	0.1%
Covered Lines:	1865
Relevant Lines:	6793

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💛 - Coveralls

[robertsLando]

LGTM! Thanks @billiaz :)