Describe the bug
Pretty much the title. I've attached the data layer I am using for calculation, but this screenshot summarizes the issue.

The results are scary when "concatenate" with delimiter is used, for exampleconcatenate(str, NULL, NULL, ' | ') or count_missing(str).
Expectations
I was expecting that NULL values are going to be omitted as they do not represent countable values. This is already the case for aggregates for numeric types.
Reading the documentation, especially on the count_missing, it states only NULL values should be counted.
https://docs.qgis.org/testing/en/docs/user_manual/working_with_vector/expression.html#aggregates-functions

Function	Description
count	Returns the count of matching features
count_distinct	Returns the count of distinct values
count_missing	Returns the count of missing (null) values

How to Reproduce

1. load the attached layer
   aggr.zip
2. this is the expression I'm using to display the result. Any other aggregate function is also affected.

'\ncount(int) ' || count("int") || 
'\ncounr(str) ' || count("str") ||
'\nmissing(int) ' || count_missing("int") || 
'\nmissing(str) ' || count_missing("str")

QGIS and OS versions
All QGIS3 versions on all platforms, it is in the very core of how statistics for strings work.
Additional context
Applying the fix breaks a few existing unit test, but also might break user expressions dependent on this bug.

Once we agree this is an issue, I already have a fix on my branch here:
suricactus@27264be