Avoid NPE when no severity is supplied in PublishDiagnosticsParams.Diagnostic #13926

Job	Run time
windows-latest jdk-17 unit tests 1 / 2	2m 49s
windows-latest jdk-17 unit tests 2 / 2	2m 47s
macOS-12 jdk-17 unit tests 1 / 2	2m 51s
Gradle MacOS integration	2m 51s
macOS-12 jdk-17 unit tests 2 / 2	2m 51s
Sbt integration	2m 50s
ubuntu-latest jdk-17 unit tests 1 / 2	2m 39s
Maven integration	2m 50s
ubuntu-latest jdk-17 unit tests 2 / 2	2m 52s
Gradle integration	2m 50s
Scala CLI integration	2m 49s
Mill integration	2m 50s
Bazel integration	2m 52s
LSP integration tests	2m 50s
Scala cross tests	2m 52s
Formatting	31s
Scalafix and docs	2m 2s
Sbt-metals/scripted jdk11	2m 41s
Scala javapc tests	1m 32s
Scala 2.11 cross tests	2m 50s
MiMa	42s
	52m 41s

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