Do not call strlen() on null

[morozov]

Fixes #656.

[ktomk]

Just a suggestion to solve this issue less invasive (better maintainability) on the original code https://github.com/justinrainbow/json-schema/blob/6d04026ebc25e8ea0594a345eaa387f2ccb3b3e5/src/JsonSchema/Constraints/Constraint.php#L49

array_filter((array)$i, 'strlen') 

and no other change (maybe a space after the cast operator depending on code-style).

and while touching this area, for the calling function https://github.com/justinrainbow/json-schema/blob/f4f0c3457811006f861c466a10c6c24f3e92f9ae/src/JsonSchema/Constraints/UndefinedConstraint.php#L40

$path = $this->incrementPath($path, $i);

as $path param is null-able. but that could be on a different commit, perhaps earlier already in master, and must not go into this PR.

the build is choking on the php-cs-fixer no_superfluous_phpdoc_tags rule, I recommend to remove it from configuration as it is incompatible with the code-style.

just my 2 cents.

[andypost]

There's duplicate #666