Fixed the issue that a null value wasn't considered an optional property

without this change an optional property was such that had an empty or non empty string. Null wasn't supported
with this change we're adding null back

fixes gh-1257

spring-cloud-contract-verifier/src/test/groovy/org/springframework/cloud/contract/verifier/dsl/wiremock/WireMockGroovyDslSpec.groovy

@@ -2713,6 +2713,47 @@ class WireMockGroovyDslSpec extends Specification implements WireMockStubVerifie
 
 	}
 
+	@Issue("#1257")
+	def "should work with null request element on the client side and optional stub entry"() {
+		given:
+			Contract contractDsl =  Contract.make {
+				description("Creating user")
+				name("Create user")
+				request {
+					method 'POST'
+					url '/api/user'
+					body(
+							address: $(consumer(optional(regex(alphaNumeric()))), producer(null)),
+							name: $(consumer(optional(regex(alphaNumeric()))), producer(''))
+					)
+					headers {
+						contentType(applicationJson())
+					}
+				}
+				response {
+					status 201
+				}
+			}
+		and:
+			String wireMockStub = new WireMockStubStrategy("Test",
+					new ContractMetadata(null, false, 0, null, contractDsl), contractDsl)
+					.toWireMockClientStub()
+
+		and:
+			int port = SocketUtils.findAvailableTcpPort()
+			WireMockServer server = new WireMockServer(config().port(port))
+			server.start()
+		and:
+			stubMappingIsValidWireMockStub(wireMockStub)
+			server.addStubMapping(WireMockStubMapping.buildFrom(wireMockStub))
+		when:
+			ResponseEntity<String> entity = callWithOptionalAndEmpty(port)
+		then:
+			entity.statusCodeValue == 201
+		cleanup:
+			server?.shutdown()
+	}
+
 	WireMockConfiguration config() {
 		return new WireMockConfiguration().extensions(responseTemplateTransformer())
 	}
@@ -2732,6 +2773,13 @@ class WireMockGroovyDslSpec extends Specification implements WireMockStubVerifie
 						.body("{\"foo\":\"bar\",\"baz\":5}"), String.class)
 	}
 
+	ResponseEntity<String> callWithOptionalAndEmpty(int port) {
+		return new TestRestTemplate().exchange(
+				RequestEntity.post(URI.create("http://localhost:" + port + "/api/user"))
+						.header("Content-Type", "application/json")
+						.body("{\"foo\":null,\"name\":\"\"}"), String.class)
+	}
+
 	ResponseEntity<byte[]> callBytes(int port, File request) {
 		return new TestRestTemplate().exchange(
 				RequestEntity.put(URI.create("http://localhost:" + port + "/1"))