Fix return type for getOrSet for a promise factory

[agubler]

Type: bug

The following has been addressed in the PR:

• There is a related issue
• All code has been formatted with prettier as per the readme code style guidelines
• Unit or Functional tests are included in the PR

Description:

Fix the getOrSet typings for a promise factory to return undefined | T, with T being the return type of the promise or the generic passed to getOrSet.

Before:

/* Type: undefined */
icache.getOrSet<string>('test', () => Promise.resolve());
/* Type: undefined */
icache.getOrSet<string>('test', () => new Promise<string>(() => {}));
/* Type: undefined */
icache.getOrSet('test', () => new Promise<string>(() => {}));

After:

/* Type: string | undefined */
icache.getOrSet<string>('test', () => Promise.resolve());
/* Type: string | undefined */
icache.getOrSet<string>('test', () => new Promise<string>(() => {}));
/* Type: string | undefined */
icache.getOrSet('test', () => new Promise<string>(() => {}));