Arm backend: Guard empty cmake arg array in build_executorch (pytorch#19840)

Avoid expanding extra_cmake_args when the array is empty.

Older Bash versions on macOS treat an empty array expansion under set -u
as an unbound variable. Append the extra CMake arguments only when the
array is non-empty so the script behaves the same on Linux and macOS.

Signed-off-by: Erik Lundell <erik.lundell@arm.com>

Author: Erik-Lundell

backends/arm/scripts/build_executorch.sh

@@ -96,9 +96,12 @@ cmake_args=(
     -DEXECUTORCH_BUILD_DEVTOOLS=${build_devtools}
     -DEXECUTORCH_BUILD_ARM_ETDUMP=${build_with_etdump}
     -DEXECUTORCH_BAREMETAL_SKIP_INSTALL=OFF
-    "${extra_cmake_args[@]}"
 )
 
+if [[ ${#extra_cmake_args[@]} -gt 0 ]]; then
+    cmake_args+=("${extra_cmake_args[@]}")
+fi
+
 if [[ -n "${target_cpu}" ]]; then
     cmake_args+=(-DTARGET_CPU=${target_cpu})
 fi