Fix null value support in ContentCachingResponseWrapper

Prior to this commit, calling `setHeader` on the response wrapper would
have a separate code path for the "Content-Length" header. This did not
support calls with `null` values and would result in an exception.

This commit ensures that the cached content length value is reset in
this case and that the call is forwarded properly to the superclass.

Fixes gh-34460

spring-web/src/test/java/org/springframework/web/filter/ContentCachingResponseWrapperTests.java

@@ -270,6 +270,17 @@ class ContentCachingResponseWrapperTests {
 				.withMessageContaining(overflow);
 	}
 
+	@Test
+	void setContentLengthNull() {
+		MockHttpServletResponse response = new MockHttpServletResponse();
+		ContentCachingResponseWrapper responseWrapper = new ContentCachingResponseWrapper(response);
+		responseWrapper.setContentLength(1024);
+		responseWrapper.setHeader(CONTENT_LENGTH, null);
+
+		assertThat(response.getHeaderNames()).doesNotContain(CONTENT_LENGTH);
+		assertThat(responseWrapper.getHeader(CONTENT_LENGTH)).isNull();
+	}
+
 
 	private void assertHeader(HttpServletResponse response, String header, int value) {
 		assertHeader(response, header, Integer.toString(value));