Doctrine/Symfony "translates" empty string into a NULL - mysql

Question

So there is a form, and non-required fields, like say "first_name". When I leave this field empty, I got an error that "Column 'first_name' cannot be null", because indeed it can't. But why does Doctrine converts empty string to NULL?
But I have to questions regarding this:
Is this possible to disable it?
What's your opinion about it? Until now I was used to the fact that when a form field was empty, I simply inserted an empty string. Not NULL o_O But... somebody told me this is a principle we should follow. For me however, NULL means "there is no value", and '' (empty string) means something different - "there IS a value, but it's empty". What do you think?
p.s. I use MySql if it matters.
edit:
there is a similar question:
Symfony2 forms interpret blank strings as nulls but maybe there is a new solution as of 2016?
The only solution which work is the one from the accepted answer:
Or otherway when you have the function setRecap you can check if empty
or not set and set the value you expect.
public function setRecap($recap) {
if(!isset($recap)) {
$this->recap = ''
} else {
$this->recap = $recap;
}
}
But I have a lot of such fields, and this seems overkill to me, there must a simpler solution.
p.s. I did not try Thomas Decaux' answer below the accepted answer, because it seems hacky to me.

Answer 1

Well, as somebody suggested me, it's as simple as:
public function setUserName($userName) {
$this->userName = (string)$userName;
return $this;
}
In an entity.
Edit: plus, we need to give a default value for the variable, like:
$userName = '';

Answer 2

Frankly it would be best to allow NULL values for your first names in your DB. Doing what you did just make things more complicated.
Symfony is developped by professionals and to me that conversion seems reasonable, because an empty string isn't more interesting than NULL.
What you said above, that NULL and an empty strings are two different things, is true in a sense, but what does that mean to have an empty name (i.e. empty string)? Isn't it better to say that this guy has no known first name (i.e. NULL)?
EDIT:
In response to your first comment:
Well in that case, typecasting to string is the right approach.