fix: pass None to rule evaluation when guardrail references missing field

[valentinabojan]

Summary

• When a guardrail rule references a field that doesn't exist in the input/output data, validation now correctly passes instead of incorrectly failing
• Added early return if not fields in evaluate_word_rule, evaluate_number_rule, and evaluate_boolean_rule
• Previously, the empty field list caused the evaluator loop to be skipped, falling through to the "violation detected" return

Test plan

• Added tests for all three rule types (word, number, boolean) with missing fields
• All 36 existing tests still pass

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