Fix some bugs with PDF imports

[InAnYan]

Some bugs:

• Several steps of PDF import didn't work because of relativization. I removed it, but IDK if it's okay to do this. After that, all steps worked.
• When searching for unlinked files, PDF import didn't work at all.
• There was some bug when importing specific PDF files, when algorithm can't find title of the book. I replaced String to Optional<String> and import worked correctly.

Mandatory checks

• I own the copyright of the code submitted and I licence it under the MIT license
• [?] Change in CHANGELOG.md described in a way that is understandable for the average user (if change is visible to the user)
  - [ ] Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
  - [ ] Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[InAnYan]

I also changed log messages as IntelliJ warned that they look like duplicates. Just in case)