Added emptyString for compatibility with other Arduino cores

[MitchBradley]

Many Arduino cores, including ESP32, pico, and libretiny, implement "String emptyString" to save space. ESPAsyncWebServer requires it. I tried adding a private _emptyString to ESPAsyncWebServer but it turned into a mess, so it seems best to implement it here to align with other cores.

[Copilot]

Pull request overview

This PR adds a globally-available emptyString symbol to ArduinoCore-Linux to improve compatibility with Arduino ecosystem libraries (notably ESPAsyncWebServer) that expect the core to provide it.

Changes:

• Declare emptyString in Arduino.h.
• Define emptyString in Arduino.cpp.
• Minor formatting fix for analogWriteResolution() declaration alignment.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
ArduinoCore-Linux/cores/arduino/Arduino.h	Adds an emptyString extern declaration for cross-core compatibility.
ArduinoCore-Linux/cores/arduino/Arduino.cpp	Adds the emptyString definition in the core implementation TU.

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[Copilot]

Defining emptyString in Arduino.cpp can force the linker to pull in this entire translation unit when only emptyString is referenced (e.g., from String/third-party libs). This can increase binary size and may also pull in unrelated globals (e.g., WifiMock WiFi behind SKIP_HARDWARE_WIFI). Consider moving the definition to a minimal dedicated .cpp (or wherever the String implementation lives) to keep the dependency surface and linked code small.

[MitchBradley]

It seems to me that this module is already pretty small, and very likely to be linked anyway since it defines millis() and delay() which are very commonly used. This Arduino core is for use on host machines which typically have lots of memory compared to the footprint of this compilation unit. If the maintainer would like for me to move it to a dedicated .cpp, I will do so, but at first glance it doesn't seem worthwhile.

[pschatzmann]

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

[MitchBradley]

Also LibreTiny https://github.com/search?q=repo%3Alibretiny-eu%2Flibretiny%20emptyString&type=code. Arduino-Emulator support in ESPAsyncWebServer is progressing. The other key component https://github.com/MitchBradley/PosixAsyncTCP is up.