Added empty data check to getKnownRelatives selector

[Carsten89]

Description

Added empty data check to getKnownRelatives selector. So it doesn't throw an error if the product data are not there yet

Type of change

Please add an "x" into the option that is relevant:

• Bug Fix 🐛 (non-breaking change which fixes an issue)
• Enhancement 🚀 (non-breaking change which adds functionality)
• Breaking Change 💥 (fix or feature that would cause existing functionality to not work as expected)
• Polish 💅 (Just some cleanups)
• Internal 🏠 Only relates to internal processes.

[coveralls]

Pull Request Test Coverage Report for Build 2906

• 1 of 2 (50.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.0001%) to 50.913%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
libraries/commerce/product/selectors/variants.js	1	2	50.0%

Totals
Change from base Build 2905:	-0.0001%
Covered Lines:	10032
Relevant Lines:	18665

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💛 - Coveralls