org-bibtex.el (org-bibtex-search): Don't throw an error when `org-bibtex-prefix' is nil

* org-bibtex.el (org-bibtex-search): Don't throw an error when
`org-bibtex-prefix' is nil.

TINYCHANGE

lisp/org-bibtex.el

@@ -679,7 +679,7 @@ This function relies `org-search-view' to locate results."
         (org-agenda-search-view-always-boolean t))
     (org-search-view nil
 		     (format "%s +{:%s%s:}"
-			     string org-bibtex-prefix
+			     string (or org-bibtex-prefix "")
 			     org-bibtex-type-property-name))))
 
 (provide 'org-bibtex)