Remove usage of Arduino constant emptyString and use our own instead

[mathieucarbou]

This PR is a cleanup in order to use our own empty string constant instead of Arduino one.

This will also help support HOST as done in PR #416

[mathieucarbou]

@MitchBradley FYI

[Copilot]

Pull request overview

This PR replaces uses of the Arduino emptyString constant with a project-owned asyncsrv::empty constant.

Changes:

• Updated default arguments across response/request APIs to use asyncsrv::empty.
• Replaced emptyString assignments/returns throughout request parsing and authentication helpers.
• Updated examples to return/assign asyncsrv::empty instead of emptyString.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 9 comments.

Show a summary per file

File	Description
src/WebResponseImpl.h	Switches AsyncBasicResponse String overload default arg to asyncsrv::empty.
src/WebRequest.cpp	Replaces emptyString in parsing/temp fields and several const String& return paths.
src/WebHandlers.cpp	Updates AsyncFileResponse content-type default argument usage.
src/WebAuthentication.cpp	Uses asyncsrv::empty for early-return empty String values on errors.
src/ESPAsyncWebServer.h	Updates multiple overload defaults and const String& accessors to use asyncsrv::empty.
src/AsyncEventSource.cpp	Returns asyncsrv::empty on allocation failure.
examples/Templates/Templates.ino	Updates template processor “no value” return to asyncsrv::empty.
examples/RequestContinuation/RequestContinuation.ino	Clears message using asyncsrv::empty.
examples/ChunkRetryResponse/ChunkRetryResponse.ino	Clears trigger string using asyncsrv::empty.

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[Copilot]

Pull request overview

Copilot reviewed 10 out of 10 changed files in this pull request and generated 5 comments.

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[MitchBradley]

@mathieucarbou Thanks for addressing this. After it is merged, should I close #416 and submit a revised version, after addressing copilot's concerns?

[mathieucarbou-ibm]

@mathieucarbou Thanks for addressing this. After it is merged, should I close #416 and submit a revised version, after addressing copilot's concerns?

You just need to rebase your HOST PR on top of branch emptyString instead of main.

Also please do not forget to open a separate PR for the state machine refactoring. This one is more requires more review and testing.

Thank you very much for these !

[MitchBradley]

One thing that occurred to me is that instead of #if defined(ESP32) || defined(HOST}, it might be better to create a new macro with a name like MULTICORE, so one could write #ifdef MULTICORE and put the "ESP32 || HOST" logic in one place.