wrapValueWithQuotes accepts nulls

without this change NPE is thrown

fixes gh-2117

spring-cloud-contract-verifier/src/main/java/org/springframework/cloud/contract/verifier/util/DelegatingJsonVerifiable.java

@@ -68,7 +68,10 @@ class DelegatingJsonVerifiable implements MethodBufferingJsonVerifiable {
 		return stringValue.replaceAll("\"", "\\\\\"");
 	}
 
-	private static String wrapValueWithQuotes(Object value) {
+	static String wrapValueWithQuotes(Object value) {
+		if (value == null) {
+			return null;
+		}
 		return value instanceof String ? "\"" + stringWithEscapedQuotes(value) + "\"" : value.toString();
 	}
 
@@ -198,8 +201,8 @@ class DelegatingJsonVerifiable implements MethodBufferingJsonVerifiable {
 			readyToCheck.methodsBuffer.offer(".value()");
 		}
 		else {
-			readyToCheck.appendMethodWithValue("isEqualTo",
-					value instanceof Long ? String.valueOf(value).concat("L") : String.valueOf(value));
+			readyToCheck.appendMethodWithValue("isEqualTo", value instanceof Long ? String.valueOf(value).concat("L")
+					: (value == null ? null : String.valueOf(value)));
 		}
 		return readyToCheck;
 	}

spring-cloud-contract-verifier/src/test/java/org/springframework/cloud/contract/verifier/util/DelegatingJsonVerifiableTests.java

@@ -0,0 +1,45 @@
+/*
+ * Copyright 2020-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.cloud.contract.verifier.util;
+
+import org.assertj.core.api.BDDAssertions;
+import org.junit.jupiter.api.Test;
+
+class DelegatingJsonVerifiableTests {
+
+	@Test
+	void shouldReturnNullValueForNull() {
+		String value = DelegatingJsonVerifiable.wrapValueWithQuotes(null);
+
+		BDDAssertions.then(value).isNull();
+	}
+
+	@Test
+	void shouldReturnQuotedValueForString() {
+		String value = DelegatingJsonVerifiable.wrapValueWithQuotes("hello");
+
+		BDDAssertions.then(value).isEqualTo("\"hello\"");
+	}
+
+	@Test
+	void shouldReturnStringValueForNonString() {
+		String value = DelegatingJsonVerifiable.wrapValueWithQuotes(5);
+
+		BDDAssertions.then(value).isEqualTo("5");
+	}
+
+}