Arm backend: Fix mypy warning in runner_utils.py (pytorch#15634)

Add a return None if elf_path is not set.

Signed-off-by: per.held@arm.com

Author: perheld

backends/arm/test/runner_utils.py

@@ -714,7 +714,9 @@ def assert_elf_path_exists(elf_path):
         )
 
 
-def get_elf_path(target_board: str, use_portable_ops: bool = False):
+def get_elf_path(target_board: str, use_portable_ops: bool = False) -> str:
+    elf_path = ""
+
     if target_board not in VALID_TARGET:
         raise ValueError(f"Unsupported target: {target_board}")
 
@@ -729,14 +731,13 @@ def get_elf_path(target_board: str, use_portable_ops: bool = False):
             f"arm_semihosting_executor_runner_{portable_ops_str}{target_board}",
             "arm_executor_runner",
         )
-        assert_elf_path_exists(elf_path)
     elif target_board == "vkml_emulation_layer":
         elf_path = os.path.join(
             f"arm_test/arm_executor_runner_{portable_ops_str}vkml",
             "executor_runner",
         )
-        assert_elf_path_exists(elf_path)
 
+    assert_elf_path_exists(elf_path)
     return elf_path