Preserve existing zero-like variables

[tanakahisateru]

I expected that variables in subsequent loading never overwrite existing one. But zero-like values are overwritten.

1st load:

MUST_BE_ZERO=0
MUST_BE_NULL=null

2nd load (with skipExistings):

MUST_BE_ZERO=1
MUST_BE_NULL=1

Unexpected result:

getenv('MUST_BE_ZERO') === "1"
$_ENV['MUST_BE_NULL'] === 1

I think that users want force disabling some option (enabled as default) in OS environment settings.

[tanakahisateru]

To research return type of getenv() and apache_getenv() below links are useful:

https://github.com/php/php-src/blob/d3bc8beb4ff9e00c3742ada32788239c3f7988e0/ext/standard/basic_functions.c#L4045
https://github.com/php/php-src/blob/c8aa6f3a9a3d2c114d0c5e0c9fdd0a465dbb54a5/sapi/apache2handler/php_functions.c#L288