chimera: fix reading of empty tags

modules/chimera/src/main/java/org/dcache/chimera/FsSqlDriver.java

@@ -1507,7 +1507,8 @@ int getTag(FsInode inode, String tagName, byte[] data, int offset, int len)
                           /* some databases (hsqldb in particular) fill a full record for
                            * BLOBs and on read reads a full record, which is not what we expect.
                            */
-                          return in.readNBytes(data, offset,
+                          // If tag is not set or NULL, then getBinaryStream will return null.
+                          return in == null ? 0 : in.readNBytes(data, offset,
                                 Math.min(len, (int) rs.getLong("isize")));
                       } catch (IOException e) {
                           throw new LobRetrievalFailureException(e.getMessage(), e);

modules/chimera/src/test/java/org/dcache/chimera/JdbcFsTest.java

@@ -1648,6 +1648,16 @@ public void testPushTag() throws Exception {
         assertArrayEquals(new String[]{tagName}, _fs.tags(d));
     }
 
+    @Test
+    public void testReadNullTag() throws Exception {
+
+        FsInode top = _rootInode.mkdir("top");
+        _fs.createTag(top, "aTag");
+
+        int n = _fs.getTag(top, "aTag", new byte[10], 0, 10);
+        assertThat("Tag without content should return zero bytes", n, is(0));
+    }
+
     @Test
     public void testTashTimestampOnRemove() throws Exception {
         final String name = "testTashTimestampOnRemove";