fix(snippet create): use filename when path missing

Description

When creating a snippet if a path was not passed but the --title and --filename flags were passed, the cli would hang.

Now we use the --filename as the path, as indicated by the existing error message (https://gitlab.com/gitlab-org/cli/-/blob/fd155babc724b510cab252c0c592120524381e86/commands/snippet/create/create.go#L61-61)

Related Issues

Resolves #7601 (closed)

How has this been tested?

Manually tested below, test added.

Screenshots (if appropriate):

Before (hangs):

After:

Types of changes

• 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)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap