@Test
public void testGetFillValue_noFillValuePresent() {

    final Variable variable = mock(Variable.class);
    when(variable.findAttribute("_FillValue")).thenReturn(null);
    final Number fillValue = NcTools.getFillValue(variable);
    assertNull(fillValue);
    verify(variable, times(1)).findAttribute("_FillValue");
    verifyNoMoreInteractions(variable);
}