fix(copystructure): handle nil elements in slice copying

When copying slices containing nil interface{} elements, the copyValue
function would panic with 'reflect: call of reflect.Value.Set on zero
Value'. This occurred because reflect.ValueOf(nil) returns a zero Value
that cannot be set.

This issue was introduced in v4.1.0 when replacing mitchellh/copystructure
with an internal implementation. The fix mirrors the existing nil handling
logic used for map values.

Fixes helm template panic when processing charts with YAML like:
  extraArgs:
    -

Added test case to verify slice elements with nil values are properly
handled during deep copy operations.

Signed-off-by: Philipp Born <git@pborn.eu>

internal/copystructure/copystructure.go

@@ -89,7 +89,15 @@ func copyValue(original reflect.Value) (any, error) {
 		}
 		copied := reflect.MakeSlice(original.Type(), original.Len(), original.Cap())
 		for i := 0; i < original.Len(); i++ {
-			val, err := copyValue(original.Index(i))
+			elem := original.Index(i)
+
+			// Handle nil values in slices (e.g., interface{} elements that are nil)
+			if elem.Kind() == reflect.Interface && elem.IsNil() {
+				copied.Index(i).Set(elem)
+				continue
+			}
+
+			val, err := copyValue(elem)
 			if err != nil {
 				return nil, err
 			}

internal/copystructure/copystructure_test.go

@@ -113,6 +113,21 @@ func TestCopy_Slice(t *testing.T) {
 		input[0]["key1"] = "modified"
 		assert.Equal(t, "value1", resultSlice[0]["key1"])
 	})
+
+	t.Run("slice with nil elements", func(t *testing.T) {
+		input := []any{
+			"value1",
+			nil,
+			"value2",
+		}
+		result, err := Copy(input)
+		require.NoError(t, err)
+
+		resultSlice, ok := result.([]any)
+		require.True(t, ok)
+		assert.Equal(t, input, resultSlice)
+		assert.Nil(t, resultSlice[1])
+	})
 }
 
 func TestCopy_Map(t *testing.T) {