[FIX]t-field wrong outputs when precision is 0

qweb FieldConverter has wrong value_to_html output, it should accept 0 as valid precision

Impacted versions:
7.0 and above

Steps to reproduce:
1. Go to Setting->Database Structure->Decimal Accuracy,Select Account,then set Digits to 0
2.Go back to Sale Order, print a [Quotation/Order],check the Amount or tax amount fields,the format and numeric is wrong , it show amount as amount / 1000

Current Behavior:
it show amount as amount / 1000