fix builtin not found err handling

[gertd]

Fix builtins that return singleton results when gRPC is a NOT_FOUND error, return an ast.NullTerm instead of an error.
The error is returned in the trace fail notes

[github-actions]

Pull Request Test Coverage Report for Build 7177056795

• 11 of 18 (61.11%) changed or added relevant lines in 4 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.7%) to 40.195%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
builtins/edge/ds/identity.go	2	3	66.67%
builtins/edge/ds/object.go	3	5	60.0%
builtins/edge/ds/relation.go	3	5	60.0%
builtins/edge/ds/user.go	3	5	60.0%

Files with Coverage Reduction	New Missed Lines	%
builtins/edge/ds/identity.go	1	84.21%

Totals
Change from base Build 7166513616:	0.7%
Covered Lines:	1441
Relevant Lines:	3585

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💛 - Coveralls

[carabasdaniel]

👍