GridFlow fixes for empty container

[penguinolog]

1. No size warnings if container empty and render size set
2. Only FLOW render supported for empty container

Fix: #900

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

[penguinolog]

not full sizing

[coveralls]

Pull Request Test Coverage Report for Build 9462520221

Details

• 9 of 9 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 74.741%

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Totals
Change from base Build 9442702303:	0.02%
Covered Lines:	9412
Relevant Lines:	12740

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💛 - Coveralls