fix: add argument node checks

Closes #29

Author: xfumihiro

rules/__tests__/prefer_to_be_null.test.js

@@ -6,7 +6,7 @@ const rules = require('../../').rules;
 const ruleTester = new RuleTester();
 
 ruleTester.run('prefer_to_be_null', rules['prefer-to-be-null'], {
-  valid: ['expect(null).toBeNull();'],
+  valid: ['expect(null).toBeNull();', 'expect(null).toEqual();'],
 
   invalid: [
     {

rules/util.js

@@ -26,22 +26,26 @@ const expectToBeCase = (node, arg) =>
   !(expectNotCase(node) || expectResolveCase(node) || expectRejectCase(node)) &&
   expectCase(node) &&
   methodName(node) === 'toBe' &&
+  argument(node) &&
   argument(node).value === arg;
 
 const expectNotToBeCase = (node, arg) =>
   expectNotCase(node) &&
   methodName2(node) === 'toBe' &&
+  argument2(node) &&
   argument2(node).value === arg;
 
 const expectToEqualCase = (node, arg) =>
   !(expectNotCase(node) || expectResolveCase(node) || expectRejectCase(node)) &&
   expectCase(node) &&
   methodName(node) === 'toEqual' &&
+  argument(node) &&
   argument(node).value === arg;
 
 const expectNotToEqualCase = (node, arg) =>
   expectNotCase(node) &&
   methodName2(node) === 'toEqual' &&
+  argument2(node) &&
   argument2(node).value === arg;
 
 const expectToBeUndefinedCase = node =>