Avoid parsing Text Plugin metadata b"{}" (#5972)

## Motivation for features / changes
There is a library that emits b"{}" as the plugin's metadata, which
triggers 500 errors in the Text Plugin server code
([b/250789176](https://b.corp.google.com/issues/250789176)):

```
google3.net.proto2.python.public.message.DecodeError: Error parsing message with type 'tensorboard.TextPluginData'
```

## Technical description of changes
This commit handles the edge case by returning a TextPluginData default
proto without parsing the metadata.

## Screenshots of UI changes
- For the edge case that feeds in b"{}", users will see something like
<img width="1213" alt="image"
src="https://user-images.githubusercontent.com/88216042/195389659-a75660ce-01e4-4ecd-ace3-19b11cadfa07.png">
rather than an "Internal server error" page.

## Detailed steps to verify changes work correctly (as executed by you)
Open the browser and enter the TextPlugin metadata endpoint of an
experiment that generates the invalid metadata, e.g.
`experiment/4725997998739972179/data/plugin/text/tags` and you should
see the resulting valid metadata.

Author: qihach64

tensorboard/plugins/text/metadata.py

@@ -53,6 +53,9 @@ def parse_plugin_metadata(content):
     """
     if not isinstance(content, bytes):
         raise TypeError("Content type must be bytes")
+    if content == b"{}":
+        # Old-style JSON format. Equivalent to an all-default proto.
+        return plugin_data_pb2.TextPluginData()
     result = plugin_data_pb2.TextPluginData.FromString(content)
     if result.version == 0:
         return result