Fix Iconv::substr nullable

[Moln]

Q	A
Documentation	no
Bugfix	yes
BC Break	yes
New Feature	no
RFC	no
QA	yes

Description

Fix iconv_substr() expects parameter 3 to be int

PHP: 7.4.25

$utf8StrWrapper = \Laminas\Stdlib\StringUtils::getWrapper('UTF-8');
$utf8StrWrapper->substr('test');
// TypeError: iconv_substr() expects parameter 3 to be int, null given 

[Ocramius]

Requires test additions

[Ocramius]

Since this is a bugfix, it should perhaps target 3.6.x?

[Moln]

Since this is a bugfix, it should perhaps target 3.6.x?

Close and will re-create PR for 3.6.x.

[Ocramius]

Moved to #37