Ensure method & requestURI are non-null in MHSR

Prior to this commit it was possible for the method and requestURI
fields in MockHttpServletRequest to be set to null.

This commit ensures that the method and requestURI fields are internally
stored as empty strings if the user sets them to a null value.

Issue: SPR-10643

spring-test/src/main/java/org/springframework/mock/web/MockHttpServletRequest.java

@@ -249,8 +249,8 @@ public class MockHttpServletRequest implements HttpServletRequest {
 	 */
 	public MockHttpServletRequest(ServletContext servletContext, String method, String requestURI) {
 		this.servletContext = (servletContext != null ? servletContext : new MockServletContext());
-		this.method = method;
-		this.requestURI = requestURI;
+		this.method = (method == null ? "" : method);
+		this.requestURI = (requestURI == null ? "" : requestURI);
 		this.locales.add(Locale.ENGLISH);
 	}
 
@@ -859,7 +859,7 @@ public class MockHttpServletRequest implements HttpServletRequest {
 	}
 
 	public void setMethod(String method) {
-		this.method = method;
+		this.method = (method == null ? "" : method);
 	}
 
 	@Override
@@ -937,7 +937,7 @@ public class MockHttpServletRequest implements HttpServletRequest {
 	}
 
 	public void setRequestURI(String requestURI) {
-		this.requestURI = requestURI;
+		this.requestURI = (requestURI == null ? "" : requestURI);
 	}
 
 	@Override