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The || (double pipe) is string concatenation which returns null because NULL doesn't contain a state or value so it can't know what to do.

QgsExpression follows the normal SQL rules with the final result being NULL. Postgres and TSQL (and the others) all follow this rule.

DECLARE @string1 as varchar(max), @string2 as varchar(max)
SET @string1 = 'Hello'
SET @string2 = NULL

SELECT @string1 + @string2

Results in NULL

Python also doesn't like it:

>>> string1 = "Hello" 
>>> string2 = None
>>> string1 + string2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: cannot concatenate 'str' and 'NoneType' objects

There is a new format() function that is cleaner then doing || all over the place. We could add NULL to '' replacement in there.