mgr/telemetry: dict.pop() errs on nonexistent key

I suggested this in a review, but failed to correctly understand
what happens when the key doesn't exist.  Test for it.

Signed-off-by: Dan Mick <dan.mick@redhat.com>

src/pybind/mgr/telemetry/module.py

@@ -336,7 +336,8 @@ class Module(MgrModule):
 
             # anonymize the smartctl report itself
             for k in ['serial_number']:
-                m.pop(k)
+                if k in m:
+                    m.pop(k)
 
             res[anon_devid] = m
         return res