minor improvements in env viz

[martin-henz]

• avoid pushing marker if agenda is empty
• remove PUSH_UNDEFINED_IF_NEEDED
• distinguish value-producing loop bodies; insert UNDEFINED only for non-value-producing loop bodies
• avoid pushing environment if agenda is empty

[coveralls]

Pull Request Test Coverage Report for Build 6214059903

• 7 of 7 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.002%) to 82.856%

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Totals
Change from base Build 6184586449:	0.002%
Covered Lines:	10886
Relevant Lines:	12705

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💛 - Coveralls