substitutions equal nothing does not work

If I write this in the substitutions ...

[cb:if cb_my_field=""] test [/cb:if]

... this won't work. But the field is really empty. How can I check if a field is empty?

The field probably isn't really empty. Check the _comprofiler database to see its exact value. If it's a checkbox for example the values can only even be 0 or 1 for example. It also depends on where the substitution is being used as it may not be substituting for the user you're expecting.

Thanks for your answer.
I have checked the database... and the fields of the users are really empty! The field is a normal textfield. I wrote this for the layout value of the userlist. it is strange, because with this [cb:if cb_my_field=""] test [/cb:if] nothing was shown... even fields which previously displayed a content no longer appears! Why? But when I test it with !="" then the fields which are not empty shows "test"!

You're overriding the output of the field and you're only overriding empty output. You need to handle both cases. Example as follows.

[cb:if FIELD_NAME=""]EMPTY[/cb:if]
[cb:if FIELD_NAME!=""][value][/cb:if]

As it's a text field even a space counts as a character and won't be empty. You also need to ensure you're editing the correct layout. Specifically you're probably wanting to edit exclusively the Profile Value Layout.

Thanks for your answer. Now I have tested it with a fresh install on xampp. I create a field "cb_field" and at the same time I write
[cb:if cb_field=""]empty[/cb:if]
[cb:if cb_feld!=""][value][/cb:if]

in the Profile Value Layout. So the field is empty for all users at this moment. But the field shows only a "-" in profile view. Because this is the default preference for empty fields in the configuration. That means it does not work

Community Builder is the version 2.0.4! Was this a bug in this version? I cant update because I had to much changes in the code!

I've no idea if it's bugged in 2.0.4, but you're pretty far out of date. What do you mean you changed code? Don't touch core code or you won't be able to update anymore unless you properly manage a GIT repository that'll let you diff and merge your changes correctly. CB Auto Actions makes it so you really should not be touching core code at all.