Run queries that only match singletons

src/gaemstone.SourceGen/ModuleGenerator.cs

@@ -321,7 +321,10 @@ public class ModuleGenerator
 					.Append(param.IsNullable ? "iter.FieldOrEmpty" : "iter.Field")
 					.AppendLine($"<{param.FieldType!.GetFullName()}>({param.TermIndex});");
 
-		sb.AppendLine("\t\t\tfor (var i = 0; i < iter.Count; i++) {");
+		// When the shim method is called, there's guaranteed to be at least one match.
+		// iter.Count may be 0 when there's no variable entities that can be matched.
+		// For example for a query that only matches singleton entities.
+		sb.AppendLine("\t\t\tfor (var i = 0; i == 0 || i < iter.Count; i++) {");
 
 		sb.Append($"\t\t\t\t{method.Name}");
 		if (method.IsGeneric) sb.Append("<T>");