Fix off-by-one error in PartEvent part count

This commit fixes an off-by-one error in the
PartEventHttpMessageReader,  so that it no longer counts empty windows.

Closes gh-32122

Author: poutsma

spring-web/src/main/java/org/springframework/http/codec/multipart/PartEventHttpMessageReader.java

@@ -155,29 +155,22 @@ public Flux<PartEvent> read(ResolvableType elementType, ReactiveHttpInputMessage
 			AtomicInteger partCount = new AtomicInteger();
 			return allPartsTokens
 					.windowUntil(t -> t instanceof MultipartParser.HeadersToken, true)
-					.concatMap(partTokens -> {
-						if (tooManyParts(partCount)) {
-							return Mono.error(new DecodingException("Too many parts (" + partCount.get() + "/" +
-									this.maxParts + " allowed)"));
-						}
-						else {
-							return partTokens.switchOnFirst((signal, flux) -> {
-								if (signal.hasValue()) {
-									MultipartParser.HeadersToken headersToken = (MultipartParser.HeadersToken) signal.get();
-									Assert.state(headersToken != null, "Signal should be headers token");
-
-									HttpHeaders headers = headersToken.headers();
-									Flux<MultipartParser.BodyToken> bodyTokens = flux.ofType(
-											MultipartParser.BodyToken.class);
-									return createEvents(headers, bodyTokens);
-								}
-								else {
+					.concatMap(partTokens -> partTokens
+							.switchOnFirst((signal, flux) -> {
+								if (!signal.hasValue()) {
 									// complete or error signal
 									return flux.cast(PartEvent.class);
 								}
-							});
-						}
-					});
+								else if (tooManyParts(partCount)) {
+									return Mono.error(new DecodingException("Too many parts (" + partCount.get() +
+											"/" + this.maxParts + " allowed)"));
+								}
+								MultipartParser.HeadersToken headersToken = (MultipartParser.HeadersToken) signal.get();
+								Assert.state(headersToken != null, "Signal should be headers token");
+
+								HttpHeaders headers = headersToken.headers();
+								return createEvents(headers, flux.ofType(MultipartParser.BodyToken.class));
+							}));
 		});
 	}
 

spring-web/src/test/java/org/springframework/http/codec/multipart/PartEventHttpMessageReaderTests.java

@@ -238,6 +238,7 @@ void tooManyParts() {
 		Flux<PartEvent> result = reader.read(forClass(PartEvent.class), request, emptyMap());
 
 		StepVerifier.create(result)
+				.assertNext(form(headers -> assertThat(headers).isEmpty(), "This is implicitly typed plain ASCII text.\r\nIt does NOT end with a linebreak."))
 				.expectError(DecodingException.class)
 				.verify();
 	}