core: EvalWriteOutput discard unknown values before writing state

The state only deals in wholly-known values, so here we null out any
unknowns for storage in state. This is okay because we subsequently write
the original, possibly-unknown value into the plan and the expression
evaluator will prefer to use this if present, allowing the unknown values
to properly propagate into other expressions in the calling module.

terraform/eval_output.go

@@ -68,7 +68,11 @@ func (n *EvalWriteOutput) Eval(ctx EvalContext) (interface{}, error) {
 	}
 
 	if val.IsKnown() && !val.IsNull() {
-		state.SetOutputValue(addr, val, n.Sensitive)
+		// The state itself doesn't represent unknown values, so we null them
+		// out here and then we'll save the real unknown value in the planned
+		// changeset below, if we have one on this graph walk.
+		stateVal := cty.UnknownAsNull(val)
+		state.SetOutputValue(addr, stateVal, n.Sensitive)
 	} else {
 		state.RemoveOutputValue(addr)
 	}