GetDirectoryName fixed for path with no slash

[dgGC5]

Description

I have fixed the bug, when the -1 returned from method LastIndexOf() was a second parameter of method Substring, which obviosuly gave an error.

Motivation and Context

I have checked the proposed change of behaviour with original C# and it seems to work as I propose.
Please check my snippet here: https://dotnetfiddle.net/ZuRy2E

How Has This Been Tested?

Screenshots

Types of changes

• Improvement (non-breaking change that improves a feature, code or algorithm)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Config and build (change in the configuration and build system, has no impact on code or features)
• Dependencies (update dependencies and changes associated, has no impact on code or features)
• Unit Tests (work on Unit Tests, has no impact on code or features)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

By the way:
Link https://github.com/nanoframework/.github/blob/main/CONTRIBUTING.md is not working for me (firefox).

[dnfadmin]

All CLA requirements met.

[josesimoes]

LGTM!