[common] Add Arduino emptyString declaration

builder/utils/cores.py

@@ -51,7 +51,7 @@ def env_add_core_sources(env: Environment, queue, name: str, path: str) -> bool:
             "+<compat/*.c*>",
             "+<port/*.c*>",
             "+<posix/*.c>",
-            "+<wiring/*.c>",
+            "+<wiring/*.c*>",
             "+<wraps/*.c>",
         ],
         includes=[

cores/common/arduino/src/wiring/wiring_compat.cpp

@@ -7,3 +7,6 @@ String ipToString(const IPAddress &ip) {
 	sprintf(szRet, "%hhu.%hhu.%hhu.%hhu", ip[0], ip[1], ip[2], ip[3]);
 	return String(szRet);
 }
+
+// global empty string to allow returning const String& with nothing
+const String emptyString;

cores/common/arduino/src/wiring/wiring_compat.h

@@ -35,6 +35,7 @@ extern "C" {
 
 #ifdef __cplusplus
 String ipToString(const IPAddress &ip);
+extern const String emptyString;
 #endif
 
 #ifdef __cplusplus