Fixed JsonVariant::isNull() not returning true after set((char*)0)

src/ArduinoJson/Strings/ConstRamStringAdapter.hpp

@@ -29,6 +29,7 @@ class ConstRamStringAdapter {
   }
 
   size_t size() const {
+    if (!_str) return 0;
     return strlen(_str);
   }
 

src/ArduinoJson/Strings/FlashStringAdapter.hpp

@@ -33,6 +33,7 @@ class FlashStringAdapter {
   }
 
   size_t size() const {
+    if (!_str) return 0;
     return strlen_P(reinterpret_cast<const char*>(_str));
   }
 

src/ArduinoJson/Strings/SizedFlashStringAdapter.hpp

@@ -29,7 +29,7 @@ class SizedFlashStringAdapter {
   }
 
   size_t size() const {
-    return strlen_P(reinterpret_cast<const char*>(_str));
+    return _size;
   }
 
   bool isStatic() const {

src/ArduinoJson/Strings/SizedRamStringAdapter.hpp

@@ -30,7 +30,7 @@ class SizedRamStringAdapter {
   }
 
   size_t size() const {
-    return strlen(reinterpret_cast<const char*>(_str));
+    return _size;
   }
 
   bool isStatic() const {