scripts/symbolize.py: Fix crash when .elf file not found

The script will crash if the xxx.elf file cannot be found.

```
TypeError: expected str, bytes or os.PathLike object, not NoneType
```

This commit add check for None value.

Signed-off-by: Kun Lai <me@imlk.top>
Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
diff --git a/scripts/symbolize.py b/scripts/symbolize.py
index e924de7..2a78434 100755
--- a/scripts/symbolize.py
+++ b/scripts/symbolize.py
@@ -262,6 +262,8 @@
         if elf_name is None:
             return ''
         elf = self.get_elf(elf_name)
+        if elf is None:
+            return ''
         cmd = self.arch_prefix('nm', elf)
         if not reladdr or not elf or not cmd:
             return ''
@@ -303,6 +305,8 @@
         if elf_name is None:
             return ''
         elf = self.get_elf(elf_name)
+        if elf is None:
+            return ''
         cmd = self.arch_prefix('objdump', elf)
         if not reladdr or not elf or not cmd:
             return ''