fix(v3/ios): empty string from a bound method no longer crashes the app (#5626)

The iOS asset response writer guarded the body pointer with 'buf != nil'
instead of its length. A bound Go method returning "" produces a zero-length
(but non-nil) response body, so '&buf[0]' panicked with index-out-of-range and
aborted the Go runtime (SIGABRT). Guard on len(buf) instead — matching the
darwin/desktop writers — and pass a nil pointer + length 0 for an empty body
(a valid empty NSData on the C side).

Reproduced and verified fixed on the iOS simulator.

Author: leaanthony

v3/UNRELEASED_CHANGELOG.md

@@ -23,6 +23,7 @@ After processing, the content will be moved to the main changelog and this file
 
 ## Fixed
 <!-- Bug fixes -->
+- Fix an iOS crash (SIGABRT) when a bound Go service method returns an empty string. The iOS asset response writer guarded the body pointer with `buf != nil` instead of its length, so a zero-length body made `&buf[0]` panic; it now guards on length, matching the desktop writers
 
 ## Deprecated
 <!-- Soon-to-be removed features -->

v3/internal/assetserver/webview/responsewriter_ios.go

@@ -103,7 +103,11 @@ func (rw *responseWriter) Write(buf []byte) (int, error) {
 
 	var content unsafe.Pointer
 	var contentLen int
-	if buf != nil {
+	// Guard on length, not just nil: a non-nil but empty slice (e.g. the body of
+	// a bound method that returned "") would make &buf[0] panic with an
+	// index-out-of-range, aborting the Go runtime. An empty body is valid — pass
+	// a nil pointer with length 0, which yields an empty NSData on the C side.
+	if len(buf) != 0 {
 		content = unsafe.Pointer(&buf[0])
 		contentLen = len(buf)
 	}