Project expenses : 0 cost is considered NULL

Hello,

today, something fantastic happened : we forecasted an expense (with planned value etc ...) for some components and we should have paid for it ... but the manufacturer gave it to us, as demonstration/testing samples (wooohooo \o/ )

So I wanted to fill the real expense cost : 0€ ... but Projeqtor sees it as a null value.
This is annoying because it’s not null, it is precisely 0. The expense isn’t cancelled and has been properly filled (null necessarily stands for « this data still has no value »).
I savagely modified the database (setting a 0 value and the realDate) : the gui properly updates, but when I click «save», it brings back the null value (and updates the DB with null).

many thanks for your answer.
For my better understanding, can you tell me how to start looking for it in the code ?

to be perfectly honest, I would like to understand why a null coalescing operator could not solve the issue, is it to maintain same code with several versions of PHP ? another reason ?
... I am not a PHP developper, so I am just trying to understand.