getNullableType: fix TS signature (#2460)

partial revert of 23b427c#diff-15f6b6c92ff954f80995d31726f89851L477
Reported by @yaacovCR here:
yaacovCR/graphql-tools-fork#40 (comment)

Author: IvanGoncharov

src/type/definition.d.ts

@@ -241,8 +241,10 @@ export function isNullableType(type: any): type is GraphQLNullableType;
 export function assertNullableType(type: any): GraphQLNullableType;
 
 export function getNullableType(type: void): undefined;
+export function getNullableType<T extends GraphQLNullableType>(type: T): T;
 export function getNullableType<T extends GraphQLNullableType>(
-  type: GraphQLNonNull<T> | T,
+  // eslint-disable-next-line @typescript-eslint/unified-signatures
+  type: GraphQLNonNull<T>,
 ): T;
 
 /**

src/type/definition.js

@@ -474,7 +474,8 @@ export function assertNullableType(type: mixed): GraphQLNullableType {
 
 /* eslint-disable no-redeclare */
 declare function getNullableType(type: void | null): void;
-declare function getNullableType<T>(type: GraphQLNonNull<T> | T): T;
+declare function getNullableType<T: GraphQLNullableType>(type: T): T;
+declare function getNullableType<T>(type: GraphQLNonNull<T>): T;
 export function getNullableType(type) {
   /* eslint-enable no-redeclare */
   if (type) {