Fix handling of array shapes without keys

[mspirkov]

Problem

Arrays of the form array{string, string, string} are converted to array{: string, : string, : string}.

Solution

The solution consists of two parts:

1. Do not convert null to an empty string in TypeResolver.
2. When the key is an empty string, do not display it in ShapeItem. This is an additional safety net for the future.

[jaapio]

Thanks for your contribution!

[jaapio]

https://github.com/phpDocumentor/TypeResolver/releases/tag/1.10.1

[mspirkov]

That's the speed. Thank you!