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-spec/src/main/groovy/org/springframework/cloud/contract/spec/internal/Common.groovy

@@ -233,7 +233,7 @@ class Common {
 	}
 
 	void assertThatSidesMatch(OptionalProperty stubSide, Object testSide) {
-		assert testSide ==~ Pattern.compile(stubSide.optionalPattern())
+		assert testSide == null || testSide ==~ Pattern.compile(stubSide.optionalPattern())
 	}
 
 	void assertThatSidesMatch(Pattern pattern, String value) {
@@ -245,7 +245,7 @@ class Common {
 	}
 
 	void assertThatSidesMatch(MatchingStrategy firstSide, MatchingStrategy secondSide) {
-		if (firstSide.type == MatchingStrategy.Type.ABSENT && secondSide != MatchingStrategy.Type.ABSENT) {
+		if (firstSide.type == MatchingStrategy.Type.ABSENT && secondSide.type != MatchingStrategy.Type.ABSENT) {
 			throwAbsentError()
 		}
 	}

spring-cloud-contract-spec/src/test/groovy/org/springframework/cloud/contract/spec/internal/ContractSpec.groovy

@@ -398,4 +398,28 @@ then:
 			assertThat(contract.request.bodyMatchers.hasMatchers()).isTrue()
 			assertThat(contract.response.bodyMatchers.hasMatchers()).isTrue()
 	}
+
+	def 'should work with optional and null value of a field'() {
+		given:
+			def contract = 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
+				}
+			}
+		expect:
+			contract != null
+	}
 }