Add tasks for all the new languages

Task/Null-object/Phix/null-object.phix

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+type nullableString(object o)
+    return string(o) or o=NULL
+end type
+nullableString s
+s = "hello"
+s = NULL
+--s = 1 -- error
+--s = {1,2,3} -- error
+
+type nullableSequence(object o)
+    return sequence(o) or o=NULL
+end type
+nullableSequence q
+q = {1,2,3}
+q = "string"    -- fine (strings are a subset of sequences)
+q = NULL
+--q = 1         -- error