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

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

@@ -75,6 +75,19 @@ class ServletServerHttpResponseTests {
 		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";