Filter out None for exceptions's message_parameter (#680)

`string.Formatter().parse(message_format)` will generate None in
field_name.
Filter it out since it may introduce issue for jsonify.

Another option is using regular expressions,
Here's a comparison of both options from gpt:
Using string.Formatter().parse():
Pros:
- The built-in string.Formatter().parse() method is designed
specifically for parsing format strings, so it is likely to handle edge
cases correctly.
- This method doesn't require an additional import.

Cons:
- The code might be less readable for people who are not familiar with
the string.Formatter() class and its .parse() method.
- It requires an additional function (iter_field_name()) and a loop to
filter out the None values.

Using re (regular expressions):
Pros:
- The code is more concise and easier to read for those familiar with
regular expressions.
- It requires fewer lines of code and directly extracts the field names,
so no additional function or loop is needed for filtering None values.

Cons:
- Regular expressions might not catch all edge cases specific to format
strings (although it will work correctly for most common cases).
- People not familiar with regular expressions might find this approach
less readable.

In summary, both methods will work for most common cases. However, if
you are mainly concerned with readability and conciseness, the regular
expression method is a better choice. On the other hand, if you
prioritize **handling all edge cases related to format strings**, the
string.Formatter().parse() method is the safer option.


Related PR: #642
# All Promptflow Contribution checklist:
- [X] **The pull request does not introduce [breaking changes].**
- [ ] **CHANGELOG is updated for new features, bug fixes or other
significant changes.**
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- [ ] **Create an issue and link to the pull request to get dedicated
review from promptflow team. Learn more: [suggested
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## General Guidelines and Best Practices
- [X] Title of the pull request is clear and informative.
- [X] There are a small number of commits, each of which have an
informative message. This means that previously merged commits do not
appear in the history of the PR. For more information on cleaning up the
commits in your PR, [see this
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### Testing Guidelines
- [ ] Pull request includes test coverage for the included changes.

---------

Co-authored-by: Robben Wang <[email protected]>

Author: huaiyan

src/promptflow/promptflow/exceptions.py

@@ -180,7 +180,9 @@ def iter_field_name():
                 return
 
             for _, field_name, _, _ in string.Formatter().parse(message_format):
-                yield field_name
+                # Last one field_name is always None, filter it out to avoid exception.
+                if field_name is not None:
+                    yield field_name
 
         # Use set to remove duplicates
         return set(iter_field_name())

src/promptflow/tests/executor/unittests/executor/test_exceptions.py

@@ -12,3 +12,4 @@ def test_exception_message(self):
         )
 
         assert ex.message == "Test exception message with parameters: test_param, <param1>."
+        assert None not in ex.message_parameters