request.php: add type check to the not empty check of a string

request.php: add type check to the not empty check of a string

The not equal comparison (<>) of a variable with an empty string
could lead to false positive results as the compare do not check
the type and thereby could not make sure that the checked variable
is a string. The usage of the not identical comparison operator
(!==) make sure that the variable is a string and not empty.