Suppress REDUNDANT_ELSE_IN_WHEN in ProtobufTaggedEncoder.encodeNull() (#2988)

^KT-76635 makes this `when` exhaustive, and without the suppress our aggregate
build for the corresponding branch fails.

Author: lunakoly

formats/protobuf/commonMain/src/kotlinx/serialization/protobuf/internal/ProtobufTaggedEncoder.kt

@@ -36,6 +36,7 @@ internal abstract class ProtobufTaggedEncoder : ProtobufTaggedBase(), Encoder, C
 
     public final override fun encodeNull() {
         if (nullableMode != NullableMode.ACCEPTABLE) {
+            @Suppress("REDUNDANT_ELSE_IN_WHEN")
             val message = when (nullableMode) {
                 NullableMode.OPTIONAL -> "'null' is not supported for optional properties in ProtoBuf"
                 NullableMode.COLLECTION -> "'null' is not supported as the value of collection types in ProtoBuf"