Fix 500 error in OPDS (#2434)

Co-authored-by: acx10 <acx10@users.noreply.github.com>

Author: acx10

booklore-api/src/main/java/com/adityachandel/booklore/service/opds/OpdsFeedService.java

@@ -604,6 +604,13 @@ private String now() {
         return DateTimeFormatter.ISO_INSTANT.format(java.time.Instant.now());
     }
 
+    private boolean hasValidFilePath(Book book) {
+        return book.getFileName() != null
+                && book.getLibraryPath() != null
+                && book.getLibraryPath().getPath() != null
+                && book.getFileSubPath() != null;
+    }
+
     private String fileMimeType(Book book) {
         if (book == null || book.getBookType() == null) {
             return "application/octet-stream";
@@ -612,7 +619,7 @@ private String fileMimeType(Book book) {
             case PDF -> "application/pdf";
             case EPUB -> "application/epub+zip";
             case FB2 -> {
-                if (book.getFileName() != null) {
+                if (hasValidFilePath(book)) {
                     ArchiveUtils.ArchiveType type = ArchiveUtils.detectArchiveType(new File(FileUtils.getBookFullPath(book)));
                     if (type == ArchiveUtils.ArchiveType.ZIP) {
                         yield "application/zip";
@@ -632,7 +639,7 @@ yield switch (book.getArchiveType()) {
                     };
                 }
 
-                if (book.getFileName() != null) {
+                if (hasValidFilePath(book)) {
                     ArchiveUtils.ArchiveType type = ArchiveUtils.detectArchiveType(new File(FileUtils.getBookFullPath(book)));
                     yield switch (type) {
                         case RAR -> "application/vnd.comicbook-rar";