Add ability to detect null string (happens when nodes gets deleted)

Author: kotl

src/FirebaseObject.cpp

@@ -54,13 +54,18 @@ float FirebaseObject::getFloat(const String& path) const {
 
 String FirebaseObject::getString(const String& path) const {
   JsonVariant variant = getJsonVariant(path);
-  if (!variant.is<const char *>()) {
+  if (!variant.is<const char *>() || isNullString(path)) {
     error_ = "failed to convert to string";
     return "";
   }
   return static_cast<const char*>(variant);
 }
 
+bool FirebaseObject::isNullString(const String& path) const {
+  JsonVariant variant = getJsonVariant(path);
+  return variant.is<const char *>() && variant.asString() == NULL;
+}
+
 JsonVariant FirebaseObject::getJsonVariant(const String& path) const {
   String key(path);
   char* start = &key[0];

src/FirebaseObject.h

@@ -43,6 +43,14 @@ class FirebaseObject {
    */
   bool getBool(const String& path = "") const;
 
+  /**
+   * Returns true if specified path is NULL string.
+   * Useful to detect tree deletions.
+   * \param optional path in the JSON object.
+   * \return result as a bool.
+   */
+  bool isNullString(const String& path = "") const;
+
   /**
    * Return the value as an int.
    * \param optional path in the JSON object.