fix output of empty generate loops

src/Language/SystemVerilog/AST/GenItem.hs

@@ -45,7 +45,7 @@ instance Show GenItem where
             x1 (show e1)
             (show c)
             x2 (show o2) (show e2)
-            (show s)
+            (if s == GenNull then "begin end" else show s)
     show (GenNull) = ";"
     show (GenModuleItem item) = show item
 

test/basic/for_decl.sv

@@ -69,6 +69,7 @@ module top;
         ;
         for (genvar n = 0; n < 32; n = n + 1)
             assign c[n] = n & 1;
+        for (genvar m = 0; m < 32; m = m + 1) begin end
     endgenerate
 
 endmodule

test/basic/for_decl.v

@@ -79,6 +79,7 @@ module top;
     generate
         for (n = 0; n < 32; n = n + 1)
             assign c[n] = n & 1;
+        for (n = 0; n < 32; n = n + 1) begin end
     endgenerate
 
 endmodule