[7.x] Make Str::endsWith return false if both haystack and needle are empty strings

[vlakoff]

Doing some investigations after looking at this Stack Overflow question, this Laravel PR and this RFC for PHP 8, I noticed the following with current Laravel code:

Str::startsWith('haystack', ''); // false
Str::startsWith('', '');         // false

Str::endsWith('haystack', '');   // false
Str::endsWith('', '');           // true, should be false

Str::contains('haystack', '');   // false
Str::contains('', '');           // false

Hence this PR to make Str::endsWith more consistent, and to add tests for this case.

Also refs #2628.

On a related note: most other languages (Java, .NET, Python...), and even future PHP 8, return true when the needle is an empty string.