ZPP: return zend_string/double directly from weak function

[Girgias]

Both of these types provide us a sentinel value that we can use (NULL and NAN).

[github-actions]

AWS x86_64 (c6id.metal)

Attribute	Value
Environment	aws
Instance type	c6id.metal
Architecture	x86_64
CPU	Intel(R) Xeon(R) Platinum 8375C CPU @ 2.90GHz, 64 cores @ 2900 MHz
CPU settings	disabled deeper C-states, disabled turbo boost, disabled hyper-threading
RAM	251 GB
Kernel	6.12.66-88.122.amzn2023.x86_64
OS	Amazon Linux 2023.10.20260202
GCC	14.2.1
Time	2026-04-12 12:20:31 UTC
Job details	https://github.com/php/php-src/actions/runs/24306603609 (Artifacts)
Changeset	https://github.com/php/php-src/compare/361084086d..493796d263

Laravel 12.11.0 demo app - 50 iterations, 50 warmups, 100 requests (sec)

PHP	Min	Max	Std dev	Rel std dev %	Mean	Mean diff %	Median	Median diff %	Skewness	Z-stat	P-value	Memory
PHP - baseline@3610840	0.38977	0.39107	0.00028	0.07%	0.39022	0.00%	0.39017	0.00%	1.079	0.000	1.000	24.85 MB
PHP - zpp-direct-return	0.38774	0.38914	0.00028	0.07%	0.38813	-0.54%	0.38810	-0.53%	1.810	8.614	0.000	24.85 MB

Symfony 2.8.0 demo app - 50 iterations, 50 warmups, 100 requests (sec)

PHP	Min	Max	Std dev	Rel std dev %	Mean	Mean diff %	Median	Median diff %	Skewness	Z-stat	P-value	Memory
PHP - baseline@3610840	0.67115	0.67527	0.00083	0.12%	0.67192	0.00%	0.67161	0.00%	2.474	0.000	1.000	25.25 MB
PHP - zpp-direct-return	0.66700	0.66862	0.00036	0.05%	0.66748	-0.66%	0.66738	-0.63%	1.026	8.614	0.000	25.24 MB

Wordpress 6.9 main page - 50 iterations, 20 warmups, 20 requests (sec)

PHP	Min	Max	Std dev	Rel std dev %	Mean	Mean diff %	Median	Median diff %	Skewness	Z-stat	P-value	Memory
PHP - baseline@3610840	0.58703	0.59429	0.00120	0.20%	0.58773	0.00%	0.58739	0.00%	4.202	0.000	1.000	25.26 MB
PHP - zpp-direct-return	0.58748	0.59043	0.00061	0.10%	0.58827	0.09%	0.58807	0.12%	2.078	-6.980	0.000	25.20 MB

bench.php - 50 iterations, 20 warmups, 2 requests (sec)

PHP	Min	Max	Std dev	Rel std dev %	Mean	Mean diff %	Median	Median diff %	Skewness	Z-stat	P-value	Memory
PHP - baseline@3610840	0.44508	0.44676	0.00042	0.09%	0.44590	0.00%	0.44593	0.00%	0.116	0.000	1.000	25.26 MB
PHP - zpp-direct-return	0.44529	0.44745	0.00044	0.10%	0.44613	0.05%	0.44605	0.03%	0.529	-2.327	0.020	25.20 MB