[12.x] Add unlessEmpty and unlessNotEmpty methods to Stringable

[mvpopuk]

Purpose

Adds unlessEmpty() and unlessNotEmpty() methods to Stringable to bring it to API parity with Collection, which already has these methods.

Current State

Collection has:

• unless($value, $callback, $default) (via Conditionable trait)
• unlessEmpty($callback, $default)
• unlessNotEmpty($callback, $default)

Stringable has:

• unless($value, $callback, $default) (via Conditionable trait)
• unlessEmpty($callback, $default) - MISSING
• unlessNotEmpty($callback, $default) - MISSING

This PR adds the missing two methods to complete the API symmetry.

Changes

• Added unlessEmpty($callback, $default) method to Stringable
• Added unlessNotEmpty($callback, $default) method to Stringable
• Added comprehensive tests for both methods

Example Usage

Before:

// Awkward - checking if empty
Str::of($input)->unless($input === '', fn($s) => $s->append('suffix'))

// Double negation
Str::of($input)->unless(! $input, fn($s) => $s->upper())

After

// Clean and expressive
Str::of($input)->unlessEmpty(fn($s) => $s->append('suffix'))

Str::of($input)->unlessNotEmpty(fn($s) => $s->upper())

Real-world example

// Apply formatting unless string is empty
$name = Str::of($userInput)
    ->trim()
    ->unlessEmpty(fn($s) => $s->title())
    ->unlessEmpty(fn($s) => $s->append('.'), fn($s) => $s->append('N/A')); 

Benefits

• API consistency: Matches Collection's unlessEmpty and unlessNotEmpty methods
• Better readability: More expressive than unless(empty()) or unless(! empty())
• Completes the API: Stringable already has whenEmpty and whenNotEmpty, now has the unless variants
• Follows Laravel patterns: Collection has had these methods for years

Backwards Compatibility

• Fully backwards compatible - pure addition with no breaking changes.

Related

• Collection has had unlessEmpty and unlessNotEmpty since Laravel 5.x
• Stringable already has unless() via Conditionable trait
• Stringable already has whenEmpty() and whenNotEmpty()
• This completes the conditional API symmetry

[taylorotwell]

I frankly find these methods so hard to even parse in my brain 😅 let's just recommend people use whenEmpty and whenNotEmpty. The double negative of unlessNotEmpty is really hard to read imo and I don't want to encourage it.