mfg_inspector output broken.
Traceback (most recent call last):
  File "C:\\Users\\CameraLab02\\venv\\lib\\site-packages\\openhtf__init__.py", line 110, in OutputTestRecord
    output_cb(record)
  File "C:\\Users\\CameraLab02\\venv\\lib\\site-packages\\openhtf\\io\\output\\mfg_inspector.py", line 469, in **call**
    testrun = _TestRunFromTestRecord(test_record)
  File "C:\\Users\\CameraLab02\\venv\\lib\\site-packages\\openhtf\\io\\output\\mfg_inspector.py", line 334, in _TestRunFromTestRe
cord
    mangled_parameters = _ExtractParameters(record, testrun, used_parameter_names)
  File "C:\\Users\\CameraLab02\\venv\\lib\\site-packages\\openhtf\\io\\output\\mfg_inspector.py", line 260, in _ExtractParameters

```
for d in measurement.dimensions]
```

AttributeError: 'NoneType' object has no attribute 'encode'

diff --git a/openhtf/io/output/mfg_inspector.py b/openhtf/io/output/mfg_inspector.py
index 65f52bd..3fe83c4 100644
--- a/openhtf/io/output/mfg_inspector.py
+++ b/openhtf/io/output/mfg_inspector.py
@@ -256,7 +256,8 @@ def _ExtractParameters(record, testrun, used_parameter_names):
         attachment = testrun.info_parameters.add()
         attachment.name = 'multidim_%s' % name
         dims = [{
-            'uom_suffix': d.uom_suffix.encode('utf8'), 'uom_code': d.uom_code}
+            'uom_suffix': d.uom_suffix and d.uom_suffix.encode('utf8'),
+            'uom_code': d.uom_code}
             for d in measurement.dimensions]
         # Refer to the module docstring for the expected schema.
         attachment.value_binary = json.dumps({
['openhtf/io/output/mfg_inspector.py']