Bug 66425: Avoid a NullPointerException found via oss-fuzz

We try to avoid throwing NullPointerException, but it was possible
to trigger one here with a specially crafted input-file

Should fix https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=61390

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1911615 13f79535-47bb-0310-9956-ffa450edef68

poi-ooxml/src/main/java/org/apache/poi/openxml4j/opc/PackagePartName.java

@@ -201,7 +201,7 @@ public final class PackagePartName implements Comparable<PackagePartName> {
         }
 
         String uriPath = partURI.getPath();
-        if (uriPath.length() == 0
+        if (uriPath == null || uriPath.isEmpty()
                 || ((uriPath.length() == 1) && (uriPath.charAt(0) == PackagingURIHelper.FORWARD_SLASH_CHAR))) {
             throw new InvalidFormatException(
                     "A part name shall not be empty [M1.1]: "