Added emptyString for compatibility with other Arduino cores (#19)

I was double checking: it seems that only the ESP32 and the RP2040 core from Earlee Phil Hower seem to support this!

Author: MitchBradley

ArduinoCore-Linux/cores/arduino/Arduino.cpp

@@ -232,3 +232,5 @@ void analogWriteResolution(uint8_t bits) {
  * @note This is used to prevent watchdog timer resets in long-running loops
  */
 void yield() {}
+
+const String emptyString;

ArduinoCore-Linux/cores/arduino/Arduino.h

@@ -49,4 +49,8 @@ using namespace arduino;
 
 // supported by some platforms
 void analogWriteFrequency(pin_size_t pin, uint32_t freq);
-void analogWriteResolution(uint8_t bits);
+void analogWriteResolution(uint8_t bits);
+
+// ESP32-ism that is adopted by most Arduino implementations
+// and used by, at least, ESPAsyncWebServer
+extern const String emptyString;