Subtle bug that occurred selecting after a join

Outer joins add the placeholder '---' that is used as some sort of NULL
value.

However select operations on such relations would always fail, because
relations are supposed to only contain values of class rstring, and
the placeholder was added as a regular string.

As a consequence, automatic casting would always fail, and the select
would always raise an exception.

Added a new test case with this specific case in mind.

test/select_join.query

@@ -0,0 +1 @@
+σ id==3(people ⧓ skills)

test/select_join.result

@@ -0,0 +1,2 @@
+id,skill,name,chief,age
+3,C++,dean,1,33