lang/funcs/transpose: Avoid crash due to map with `null` values (#36611)

* lang/funcs/transpose: Avoid crash on nulls in input

* add changelog entry

.changes/v1.11/BUG FIXES-20250303-125722.yaml

@@ -0,0 +1,5 @@
+kind: BUG FIXES
+body: 'lang/funcs/transpose: Avoid crash due to map with null values'
+time: 2025-03-03T12:57:22.400359Z
+custom:
+    Issue: "36611"

internal/lang/funcs/collection.go

@@ -582,8 +582,14 @@ var TransposeFunc = function.New(&function.Spec{
 
 		for it := inputMap.ElementIterator(); it.Next(); {
 			inKey, inVal := it.Element()
+			if inVal.IsNull() {
+				return cty.MapValEmpty(cty.List(cty.String)), errors.New("input must not contain null list")
+			}
 			for iter := inVal.ElementIterator(); iter.Next(); {
 				_, val := iter.Element()
+				if val.IsNull() {
+					return cty.MapValEmpty(cty.List(cty.String)), errors.New("input list must not contain null string")
+				}
 				if !val.Type().Equals(cty.String) {
 					return cty.MapValEmpty(cty.List(cty.String)), errors.New("input must be a map of lists of strings")
 				}

internal/lang/funcs/collection_test.go

@@ -1833,6 +1833,37 @@ func TestTranspose(t *testing.T) {
 			}).WithMarks(cty.NewValueMarks("beep", "boop", "bloop")),
 			false,
 		},
+		{
+			cty.NullVal(cty.Map(cty.List(cty.String))),
+			cty.NilVal,
+			true,
+		},
+		{
+			cty.MapVal(map[string]cty.Value{
+				"test": cty.NullVal(cty.List(cty.String)),
+			}),
+			cty.NilVal,
+			true,
+		},
+		{
+			cty.MapVal(map[string]cty.Value{
+				"test": cty.ListVal([]cty.Value{cty.NullVal(cty.String)}),
+			}),
+			cty.NilVal,
+			true,
+		},
+		{
+			cty.UnknownVal(cty.Map(cty.List(cty.String))),
+			cty.UnknownVal(cty.Map(cty.List(cty.String))).RefineNotNull(),
+			false,
+		},
+		{
+			cty.MapVal(map[string]cty.Value{
+				"test": cty.ListVal([]cty.Value{cty.UnknownVal(cty.String)}),
+			}),
+			cty.UnknownVal(cty.Map(cty.List(cty.String))).RefineNotNull(),
+			false,
+		},
 	}
 
 	for _, test := range tests {