[11.x] Add some tests and improvement test coverage for Str::camel

[saMahmoudzadeh]

This PR , adds some tests to enhance test coverage.

• Check empty string

 $this->assertSame('', Str::camel(''));

• When all characters are uppercase

$this->assertSame('lARAVELPHPFRAMEWORK', Str::camel('LARAVEL_PHP_FRAMEWORK'));

• When there are leading and trailing spaces in a string.

 $this->assertSame('laravelPhpFramework', Str::camel('   laravel   php   framework   '));

• When there is a numeric character in the middle or at the beginning of the string

$this->assertSame('foo1Bar', Str::camel('foo1_bar'));
$this->assertSame('1FooBar', Str::camel('1 foo bar'));