Fix NullPointerException in Jackson2SmileDecoder

Fix uncommon case in Jackson2SmileDecoder, where a null token,
incicating a document separator in streaming mode, is followed by
NOT_AVAILABLE.

Closes gh-24009

(cherry picked from commit 5f3c7ca)

Author: poutsma

spring-web/src/main/java/org/springframework/http/codec/json/Jackson2Tokenizer.java

@@ -120,13 +120,18 @@ private Flux<TokenBuffer> endOfInput() {
 	private List<TokenBuffer> parseTokenBufferFlux() throws IOException {
 		List<TokenBuffer> result = new ArrayList<>();
 
-		while (true) {
+		// SPR-16151: Smile data format uses null to separate documents
+		boolean previousNull = false;
+		while (!this.parser.isClosed()) {
 			JsonToken token = this.parser.nextToken();
-			// SPR-16151: Smile data format uses null to separate documents
 			if (token == JsonToken.NOT_AVAILABLE ||
-					(token == null && (token = this.parser.nextToken()) == null)) {
+					token == null && previousNull) {
 				break;
 			}
+			else if (token == null ) { // !previousNull
+				previousNull = true;
+				continue;
+			}
 			updateDepth(token);
 			if (!this.tokenizeArrayElements) {
 				processTokenNormal(token, result);
@@ -167,6 +172,9 @@ private void processTokenNormal(JsonToken token, List<TokenBuffer> result) throw
 
 	private void processTokenArray(JsonToken token, List<TokenBuffer> result) throws IOException {
 		if (!isTopLevelArrayToken(token)) {
+			if (!this.parser.hasCurrentToken()) {
+				System.out.println("NO CURRENT TOKEN: " + token);
+			}
 			this.tokenBuffer.copyCurrentEvent(this.parser);
 		}