refactor(fs): use underlyingPath to determine virtual files more reliably

[nikpivkin]

Description

Previously, empty virtual files (with zero length) were not recognized as virtual, causing errors when opening or calling Stat on them. This was due to virtual files being identified based on their size. This PR changes the criteria: a file is now considered virtual if it has no underlyingPath.

We can safely rely on underlyingPath to determine whether a file is virtual because files are created in only a limited number of controlled ways:
• Writing a real file: underlyingPath is required and an error is returned if it’s missing.
• Writing a virtual file: underlyingPath is intentionally left empty.
• Creating a virtual directory: underlyingPath is also empty in this case.

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).