EAGLESIX-2712: minor changes

lib/pipeline/expressions/MapExpression.js

@@ -83,11 +83,11 @@ proto.evaluateInternal = function evaluateInternal(vars) {
 	}
 
 	if(!(inputVal instanceof Array)) {
-		throw new Error("Input to $map must be an Array, not a " + typeof inputVal + " 16883");
+		throw new Error("Uassert 16883: Input to $map must be an Array, not a " + typeof inputVal);
 	}
 
 	if(inputVal.length === 0) {
-		return [];
+		return inputVal;
 	}
 
 	// Diverge from Mongo source here, as Javascript has a builtin map operator.