Add null check for image.getFormat()

Under some conditions, an Image might have no associated format. This
will be the case for ROMIO pixel buffer e.g. created by scripts or
OMERO pyramid pixel buffer  created by the PixelService for large
images without resolutions.

src/main/java/com/glencoesoftware/omero/zarr/ZarrPixelsService.java

@@ -325,7 +325,7 @@ protected ZarrPixelBuffer createOmeNgffPixelBuffer(Pixels pixels) {
             String uri = getUri(image);
             if (uri == null) {
                 // Quick exit if we think we're OME-NGFF but there is no URI
-                if ("OMEXML".equals(image.getFormat().getValue())) {
+                if (image.getFormat() != null && "OMEXML".equals(image.getFormat().getValue())) {
                     throw new LockTimeout("Import in progress.", 15*1000, 0);
                 }
                 log.debug("No OME-NGFF root");