Fix null custom headers

[hiro-at-pieces]

No description provided.

[linux-foundation-easycla]

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[mraleph]

You need to sign CLA otherwise we can't even look your patch.

[hiro-at-pieces]

You need to sign CLA otherwise we can't even look your patch.

@mraleph I just signed EasyCLA. Let me know if you have any questions

[mosuem]

Is this to solve #736? From the discussion there, I don't believe this is the right fix. Could you elaborate?

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
grpc	None	4.2.0	4.2.0	4.2.0	✔️

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Changelog Entry ❗

Package	Changed Files
package:grpc	lib/src/server/handler.dart

Changes to files need to be accounted for in their respective changelogs.

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API leaks ⚠️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package	Leaked API symbol	Leaking sources
grpc	$1.Duration	generated/google/rpc/error_details.pb.dart::RetryInfo::retryDelay generated/google/protobuf/duration.pb.dart::Duration::clone generated/google/protobuf/duration.pb.dart::Duration::copyWith generated/google/protobuf/duration.pb.dart::Duration::create generated/google/protobuf/duration.pb.dart::Duration::createEmptyInstance generated/google/protobuf/duration.pb.dart::Duration::createRepeated generated/google/protobuf/duration.pb.dart::Duration::getDefault generated/google/rpc/error_details.pb.dart::RetryInfo::retryDelay generated/google/rpc/error_details.pb.dart::RetryInfo::ensureRetryDelay generated/google/rpc/error_details.pb.dart::RetryInfo::new::retryDelay

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