added testcase for categories with lincat C = {};

testsuite/check/lins/linsCnc.gf

@@ -0,0 +1,11 @@
+concrete linsCnc of lins = {
+
+lincat Nat =  {} ;
+
+-- we expect warnings because the lines bellow are commented out
+-- we don't expect warning for zero because Nat = {}
+
+-- lincat C = {s : Str} ;
+-- lin test = {s = "test"} ;
+
+}