autotest: Fix hostinfo docstring and callers for None
am: 873251842c

Change-Id: I8c545b96358fb1976e22a31129e1c90d9019e474
diff --git a/server/hosts/cros_host.py b/server/hosts/cros_host.py
index 86a966f..a3581df 100644
--- a/server/hosts/cros_host.py
+++ b/server/hosts/cros_host.py
@@ -313,7 +313,7 @@
 
     def get_cros_repair_image_name(self):
         info = self.host_info_store.get()
-        if info.board is None:
+        if not info.board:
             raise error.AutoservError('Cannot obtain repair image name. '
                                       'No board label value found')
         return afe_utils.get_stable_cros_image_name(info.board)
diff --git a/server/hosts/host_info.py b/server/hosts/host_info.py
index fd79462..920b563 100644
--- a/server/hosts/host_info.py
+++ b/server/hosts/host_info.py
@@ -73,7 +73,8 @@
     def board(self):
         """Retrieve the board label value for the host.
 
-        @returns: The (stripped) board label, or None if no label is found.
+        @returns: The (stripped) board label, or the empty string if no
+        label is found.
         """
         return self.get_label_value(self._BOARD_PREFIX)
 
@@ -82,7 +83,8 @@
     def model(self):
         """Retrieve the model label value for the host.
 
-        @returns: The (stripped) model label, or None if no label is found.
+        @returns: The (stripped) model label, or the empty string if no
+        label is found.
         """
         return self.get_label_value(self._MODEL_PREFIX)
 
@@ -91,8 +93,9 @@
     def os(self):
         """Retrieve the os for the host.
 
-        @returns The os (str) or None if no os label exists. Returns the first
-                matching os if mutiple labels are found.
+        @returns The os (str) or the empty string if no os label
+                exists. Returns the first matching os if mutiple labels
+                are found.
         """
         return self.get_label_value(self._OS_PREFIX)