Fix 'NoneType' object has no attribute 'startswith'

When --fuchsia-out-dir is not specified, the variable is NoneType. It
will hit "'NoneType' object has no attribute 'startswith'" error when
trying to call os.path.expanduser() with it.

Bug: 1115450
Change-Id: Iaf920b47ceb664a6ad7cd4963a016d1dfb60147e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2351934Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Commit-Queue: Chih-Hsuan Kuo <chkuo@google.com>
Auto-Submit: Chih-Hsuan Kuo <chkuo@google.com>
Cr-Commit-Position: refs/heads/master@{#798572}

build/fuchsia/device_target.py

@@ -85,7 +85,7 @@ class DeviceTarget(target.Target):
     self._port = port if port else 22
     self._system_log_file = system_log_file
     self._host = host
-    self._fuchsia_out_dir = os.path.expanduser(fuchsia_out_dir)
+    self._fuchsia_out_dir = fuchsia_out_dir
     self._node_name = node_name
     self._os_check = os_check
     self._amber_repo = None
@@ -98,6 +98,7 @@ class DeviceTarget(target.Target):
         raise Exception('Only one of "--fuchsia-out-dir" or "--ssh_config" can '
                         'be specified.')
 
+      self._fuchsia_out_dir = os.path.expanduser(fuchsia_out_dir)
       # Use SSH keys from the Fuchsia output directory.
       self._ssh_config_path = os.path.join(self._fuchsia_out_dir, 'ssh-keys',
                                            'ssh_config')