Return empty string instead of nil when linting on required

This allows lint to work in scenarios when required is used in secrets or it's output is passed to another function.
Due to lint mode no longer failing on missing value in required it is passing nil through which not all functions can accept.

Fixes #4747

Signed-off-by: Helgi Þorbjörnsson <helgith@gmail.com>

pkg/engine/engine.go

@@ -161,7 +161,8 @@ func (e *Engine) alterFuncMap(t *template.Template, referenceTpls map[string]ren
 				log.Printf("[INFO] Missing required value: %s", warn)
 				return val, nil
 			}
-			return val, fmt.Errorf(warn)
+			// Convert nil to "" in case required is piped into other functions
+			return "", fmt.Errorf(warn)
 		} else if _, ok := val.(string); ok {
 			if val == "" {
 				if e.LintMode {