Return consistent collection type for matrix variables

See gh-31483

Author: Bernie Schelberg

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/MatrixVariableMapMethodArgumentResolver.java

@@ -69,7 +69,7 @@ public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewC
 						HandlerMapping.MATRIX_VARIABLES_ATTRIBUTE, RequestAttributes.SCOPE_REQUEST);
 
 		if (CollectionUtils.isEmpty(matrixVariables)) {
-			return Collections.emptyMap();
+			return (isSingleValueMap(parameter) ? Collections.emptyMap() : new LinkedMultiValueMap<>(0));
 		}
 
 		MultiValueMap<String, String> map = new LinkedMultiValueMap<>();

spring-webmvc/src/test/java/org/springframework/web/servlet/mvc/method/annotation/MatrixVariablesMapMethodArgumentResolverTests.java

@@ -155,6 +155,18 @@ public void resolveArgumentNoParams() throws Exception {
 		assertThat(map).isEqualTo(Collections.emptyMap());
 	}
 
+	@Test
+	public void resolveMultiValueMapArgumentNoParams() throws Exception {
+
+		MethodParameter param = this.testMethod.annot(matrixAttribute().noPathVar())
+				.arg(MultiValueMap.class, String.class, String.class);
+
+		Object result = this.resolver.resolveArgument(param, this.mavContainer, this.webRequest, null);
+
+		//noinspection unchecked
+		assertThat(result).isInstanceOfSatisfying(MultiValueMap.class, map -> assertThat(map).isEmpty());
+	}
+
 	@Test
 	public void resolveArgumentNoMatch() throws Exception {
 		MultiValueMap<String, String> params2 = getVariablesFor("planes");