I want to change the invoice document title if there is no tax on the invoice. 

I tried the following:

<span t-if="o.amount_tax == '0'">Test 0</span>
<span t-if="o.amount_tax == '0.00'">Test 0.00</span>
<span t-if="o.amount_tax == 'null'">Test null</span>
<span t-if="o.amount_tax == '0.00 €'">Test €</span>
<span t-if="o.amount_tax == null">Test null ohne</span>

nothing helped to change the title altought the value of the field seems to be 0.00 €

thanks

Philipp