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

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

@@ -238,6 +238,7 @@ class PartEventHttpMessageReaderTests {
 		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();
 	}