Treat query param w/o "=" as empty string in HtmlUnitRequestBuilder

Prior to this commit, HtmlUnitRequestBuilder stored empty query
parameters declared without an equals sign as null (i.e., query
parameters such as 'error' in 'http://example.com/login?error').

This commit addresses this issue by ensuring that
HtmlUnitRequestBuilder treats all empty query parameter values as empty
strings. Consequently, query strings such as '?error' and '?error=' now
both result in 'error' being stored as an empty string.

Issue: SPR-13524

spring-test/src/test/java/org/springframework/test/web/servlet/htmlunit/HtmlUnitRequestBuilderTests.java

@@ -417,7 +417,7 @@ public class HtmlUnitRequestBuilderTests {
 		MockHttpServletRequest actualRequest = requestBuilder.buildRequest(servletContext);
 
 		assertThat(actualRequest.getParameterMap().size(), equalTo(1));
-		assertThat(actualRequest.getParameter("name"), nullValue());
+		assertThat(actualRequest.getParameter("name"), equalTo(""));
 	}
 
 	@Test