CV7000: handle NullPointerException when resetting allFiles in close()

While the reader should initialize this field as an empty ArrayList, there are
scenarios where allFiles will be null when calling close().
A use case would be a reader initialized from a memo file created with a
previous release of Bio-Formats since this field was formerly a string array
set to null by default.

components/formats-gpl/src/loci/formats/in/CV7000Reader.java

@@ -223,7 +223,11 @@ public void close(boolean fileOnly) throws IOException {
       reversePlaneLookup = null;
       extraFiles = null;
       acquiredWells.clear();
-      allFiles.clear();
+      if (allFiles != null) {
+          allFiles.clear();
+      } else {
+          allFiles = new ArrayList<String>();
+      }
     }
   }