Fix ServletResponseHttpHeaders#get null handling

ServletResponseHttpHeaders#get should be annotated with `@Nullable` and
return null instead of a singleton list containing null when there is no
content type header.

Closes gh-32362

Author: sdeleuze

spring-web/src/main/java/org/springframework/http/server/ServletServerHttpResponse.java

@@ -171,13 +171,15 @@ public String getFirst(String headerName) {
 		}
 
 		@Override
+		@Nullable
 		public List<String> get(Object key) {
 			Assert.isInstanceOf(String.class, key, "Key must be a String-based header name");
 
 			String headerName = (String) key;
 			if (headerName.equalsIgnoreCase(CONTENT_TYPE)) {
 				// Content-Type is written as an override so don't merge
-				return Collections.singletonList(getFirst(headerName));
+				String value = getFirst(headerName);
+				return (value != null ? Collections.singletonList(value) : null);
 			}
 
 			Collection<String> values1 = servletResponse.getHeaders(headerName);

spring-web/src/test/java/org/springframework/http/server/ServletServerHttpResponseTests.java

@@ -75,6 +75,19 @@ void getHeaders() {
 		assertThat(mockResponse.getCharacterEncoding()).as("Invalid Content-Type").isEqualTo("UTF-8");
 	}
 
+	@Test
+	void getHeadersWithNoContentType() {
+		this.response = new ServletServerHttpResponse(this.mockResponse);
+		assertThat(this.response.getHeaders().get(HttpHeaders.CONTENT_TYPE)).isNull();
+	}
+
+	@Test
+	void getHeadersWithContentType() {
+		this.mockResponse.setContentType(MediaType.TEXT_PLAIN_VALUE);
+		this.response = new ServletServerHttpResponse(this.mockResponse);
+		assertThat(this.response.getHeaders().get(HttpHeaders.CONTENT_TYPE)).containsExactly(MediaType.TEXT_PLAIN_VALUE);
+	}
+
 	@Test
 	void preExistingHeadersFromHttpServletResponse() {
 		String headerName = "Access-Control-Allow-Origin";