Add type checks to SFTP.php

[harrykipper]

Description

Adds a type check

Related Issue

Fixes https://github.com/owncloud/core/issues/41159

Motivation and Context

Removes errors from the log

How Has This Been Tested?

• test environment: large 1000+ users installation

Screenshots (if appropriate):

Types of changes

• [X ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Database schema changes (next release will require increase of minor version instead of patch)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Technical debt
• [ ] Tests only (no source changes)

Checklist:

• [X ] Code changes
• [ ] Unit tests added
• [ ] Acceptance tests added
• [ ] Documentation ticket raised:
• [ ] Changelog item, see TEMPLATE

[update-docs[bot]]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would create a changelog item based on your changes.

[CLAassistant]

All committers have signed the CLA.

[DeepDiver1975]

@harrykipper please bring this to a mergable condition. THX

[phil-davis]

@harrykipper commit messages need to conform to https://www.conventionalcommits.org/en/v1.0.0/

A changelog needs to be added here.

You started this PR directly from the master branch in your fork. I suggest that you use a different branch(es) for making PRs and keep the master in your fork up-to-date with core master.

[iasdeoupxe]

A changelog needs to be added here.

Can be pulled from #41309