cmake: avoid -Wnonnull warning in HAVE_FSETXATTR_5 detection

Seen in Android 21/35 CI jobs:
```
curl/CMake/CurlTests.c:315:16: warning: null passed to a callee that requires a non-null argument [-Wnonnull]
   315 |   fsetxattr(0, 0, 0, 0, 0);
       |                ^
 1 warning generated.
```
Ref: https://github.com/curl/curl/actions/runs/13460225795/job/37613494183#step:9:5978

Follow-up to bd9f9b085a #16377
Closes #16427

CMake/CurlTests.c

@@ -312,7 +312,7 @@ int main(void)
 #include <sys/xattr.h> /* header from libc, not from libattr */
 int main(void)
 {
-  fsetxattr(0, 0, 0, 0, 0);
+  fsetxattr(0, "", 0, 0, 0);
   return 0;
 }
 #endif