Use toAbsolutePath in FileProxy#downloadToPath

[MinnDevelopment]

Pull Request Etiquette

• I have checked the PRs for upcoming features/bug fixes.
• I have read the contributing guidelines.

Changes

• Internal code
• Library interface (affecting end-user code)
• Documentation
• Other: _____

Closes Issue: NaN

Description

Fixes bug with resolving parent directory of paths for FileProxy

Exception

java.lang.NullPointerException: Cannot invoke "java.nio.file.Path.getFileSystem()" because "path" is null
        at java.base/java.nio.file.Files.provider(Files.java:105)
        at java.base/java.nio.file.Files.exists(Files.java:2520)
        at net.dv8tion.jda.api.utils.FileProxy.downloadToPath(FileProxy.java:202)
        at net.dv8tion.jda.api.utils.FileProxy.downloadToPath(FileProxy.java:310)
        at net.dv8tion.jda.api.utils.FileProxy.downloadToPath(FileProxy.java:196)
        at net.dv8tion.jda.api.utils.FileProxy.downloadToPath(FileProxy.java:256)

Caused by path.getParent() being used on Paths.get("thumbnail.jpg") which is a relative path without defined parent folder.