Bug 1819783 - Avoid logging errors when dealing with records with no params. r=ahal

Under some conditions, logging fails with messages like:

```
--- Logging error ---
Traceback (most recent call last):
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.9/lib/python3.9/logging/__init__.py", line 1083, in emit
    msg = self.format(record)
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.9/lib/python3.9/logging/__init__.py", line 927, in format
    return fmt.format(record)
  File "/Users/glandium/mozilla-unified/python/mach/mach/logging.py", line 156, in format
    formatted_msg = record.msg.format(**record.params)
AttributeError: 'LogRecord' object has no attribute 'params'
```

In those cases, the record does have a message, as expected, but it's
preformatted and there are no params in the record at all, rather than
an empty dict.

Differential Revision: https://phabricator.services.mozilla.com/D171408

python/mach/mach/logging.py

@@ -118,7 +118,7 @@ class StructuredHumanFormatter(logging.Formatter):
         self.last_time = None
 
     def format(self, record):
-        formatted_msg = record.msg.format(**record.params)
+        formatted_msg = record.msg.format(**getattr(record, "params", {}))
 
         elapsed_time = (
             format_seconds(self._time(record)) + " " if self.write_times else ""