public static String substVars(String val, PropertyContainer pc1) throws ScanException {
        return substVars(val, pc1, null);
    }
@Test
    public void emptyVariableIsAccepted() throws JoranException, ScanException {
        String varName = "var"+diff;
        context.putProperty(varName, "");
        String r = OptionHelper.substVars("x ${"+varName+"} b", context);
        assertEquals("x  b", r);
    }