utils: T6658: fix write_file check in case of empty directory path (backport #3989)

[mergify]

Change Summary

Fix check if dirname empty. This is a minor error, but an annoyance after the revisions to the migration system (T6007; not yet backported), which make use of the util write_file for vyos-save-config.py, looking ahead to making config write operations atomic (T6472).

For example, one may want to save a config locally: save some-config without dirpath, which will raise the error:
FileNotFoundError: [Errno 2] No such file or directory: ''
Fix now and backport.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

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• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

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This is an automatic backport of pull request #3989 done by [Mergify](https://mergify.com).

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