Bug: Generating pdf using formatter from search results page results in blank pdf

**Describe the bug**
To get around the fact that it does not seem possible at the moment to generate a multi-lingual pdf from the search page (either labels or metadata) I have been trying to get the formatter approach to work as mentioned in [web/src/main/webapp/xslt/services/pdf/portal-present-fop.xsl](https://github.com/geonetwork/core-geonetwork/blob/4.2.x/web/src/main/webapp/xslt/services/pdf/portal-present-fop.xsl). I have a working formatter that generates results correctly when called from a browser using (eg) `http://localhost/geonetwork/api/records/UUID/formatters/myformatter?language=wel&output=pdf` but I am struggling to get this to work as the `src` in
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I have tried simply assigning the URL to `src`, and building it using `concat` statements but nothing makes any difference. When I print out the value of `src` as an `xsl:message` it is correct, and the code generates a pdf, but it is blank. I have tried debugging this in debug mode and in the web console in my browser but I can't see any errors. My record is public so there should be no authentication problems.

**Additional context**
Related issues, to do with generating multi-lingual pdfs from the search page include https://github.com/geonetwork/core-geonetwork/issues/6630 but that just refers to the language of the labels.
I have also tried to get around these problems by reverting to the traditional non-formatter approach in https://github.com/geonetwork/core-geonetwork/blob/4.2.x/web/src/main/webapp/xslt/services/pdf/portal-present-fop.xsl and adapting https://github.com/geonetwork/core-geonetwork/blob/4.2.x/web/src/main/webapp/xslt/services/pdf/metadata-fop.xsl. I can hard-code the labels to a certain extent but I'm stuck with the fact that elements like `resourceAbstractObject` only include the default value from the index and not the translations.

Any help with either approach would be much appreciated!

Start with web/src/main/webapp/xslt/services/pdf/portal-present-fop.xsl and compare the formatter URL that works in a browser with the fox:external-document src used there. Investigate why the generated PDF is blank despite the src value being correct, using the formatter endpoint and the traditional metadata-fop.xsl approach as context. Done means the search-results PDF contains the expected record content.

Written by the indexing model from the issue text.