CVE-2023-32732: gRPC connection termination issue

The vulnerability stems from missing null checks in header processing functions. The patch adds Preconditions.checkNotNull() to various toBytes/toStream methods in Metadata's inner classes. These functions process '-bin' suffixed headers and convert values using marshallers. Without null checks, invalid base64 inputs would cause NPEs during header serialization, terminating connections. The vulnerable functions are the original implementations that directly returned marshaller results without validation, as evidenced by the added null checks in the patch. All modified functions handle header value conversion and would appear in stack traces when processing malicious headers.