Allow removal of empty graphic overviews

[tylerjmchugh]

Currently an empty graphic overview cannot be removed as the match condition checks for an empty file name. This PR aims to fix this issue by removing the check for an empty file name allowing empty graphic overviews to be removed.

An example of an empty graphic overview looks like:

<gmd:graphicOverview>
    <gmd:MD_BrowseGraphic>
        <gmd:fileName gco:nilReason="missing">
            <gco:CharacterString/>
        </gmd:fileName>
    </gmd:MD_BrowseGraphic>
</gmd:graphicOverview>

Checklist

• [X] I have read the contribution guidelines
• [X] Pull request provided for main branch, backports managed with label
• [X] Good housekeeping of code, cleaning up comments, tests, and documentation
• [X] Clean commit history broken into understandable chucks, avoiding big commits with hundreds of files, cautious of reformatting and whitespace changes
• [X] Clean commit messages, longer verbose messages are encouraged
• [ ] API Changes are identified in commit messages
• [ ] Testing provided for features or enhancements using automatic tests
• [ ] User documentation provided for new features or enhancements in manual
• [ ] Build documentation provided for development instructions in README.md files
• [ ] Library management using pom.xml dependency management. Update build documentation with intended library use and library tutorials or documentation