Fix startup error. Follow up #7456

[josegar74]

Follow up of #7456. Fix startup error:

javax.servlet.ServletException: targetUri  is required in web.xml or set externally
    at org.fao.geonet.proxy.URITemplateProxyServlet.initTarget (URITemplateProxyServlet.java:179)
    at org.mitre.dsmiley.httpproxy.ProxyServlet.init (ProxyServlet.java:220)

Checklist

• I have read the contribution guidelines
• Pull request provided for main branch, backports managed with label
• Good housekeeping of code, cleaning up comments, tests, and documentation
• Clean commit history broken into understandable chucks, avoiding big commits with hundreds of files, cautious of reformatting and whitespace changes
• 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