patch 9.1.0335: String interpolation fails for List type

Problem:  String interpolation fails for List type
Solution: use implicit string(list) for string interpolation and :put =
          (Yegappan Lakshmanan)

related: #14529
closes: #14556

Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_let.vim b/src/testdir/test_let.vim
index fec0273..2aba1d3 100644
--- a/src/testdir/test_let.vim
+++ b/src/testdir/test_let.vim
@@ -696,6 +696,41 @@
   END
   call assert_equal(["let d2 = {'a': 10, 'b': 'ss', 'c': {}}"], code)
 
+  " Empty dictionary
+  let d1 = {}
+  let code =<< eval trim END
+    let d2 = {d1}
+  END
+  call assert_equal(["let d2 = {}"], code)
+
+  " null dictionary
+  let d1 = test_null_dict()
+  let code =<< eval trim END
+    let d2 = {d1}
+  END
+  call assert_equal(["let d2 = {}"], code)
+
+  " Evaluate a List
+  let l1 = ['a', 'b', 'c']
+  let code =<< eval trim END
+    let l2 = {l1}
+  END
+  call assert_equal(["let l2 = ['a', 'b', 'c']"], code)
+
+  " Empty List
+  let l1 = []
+  let code =<< eval trim END
+    let l2 = {l1}
+  END
+  call assert_equal(["let l2 = []"], code)
+
+  " Null List
+  let l1 = test_null_list()
+  let code =<< eval trim END
+    let l2 = {l1}
+  END
+  call assert_equal(["let l2 = []"], code)
+
   let code = 'xxx'
   let code =<< eval trim END
     let n = {5 +