fix: check method(node) returns truthy before trying to using results (#358)

Fixes #357

Author: G-Rath

src/rules/__tests__/prefer-to-be-null.test.js

@@ -17,6 +17,7 @@ ruleTester.run('prefer-to-be-null', rule, {
     'expect("a string").not.toMatchSnapshot();',
     "expect(something).toEqual('a string');",
     'expect(null).toBe',
+    'expect("something");',
   ],
 
   invalid: [

src/rules/__tests__/prefer-to-be-undefined.test.js

@@ -14,6 +14,7 @@ ruleTester.run('prefer-to-be-undefined', rule, {
     'expect(something).not.toBe(somethingElse)',
     'expect(something).not.toEqual(somethingElse)',
     'expect(undefined).toBe',
+    'expect("something");',
   ],
 
   invalid: [

src/rules/util.js

@@ -87,9 +87,9 @@ export const method = node => node.parent.property;
 
 export const method2 = node => node.parent.parent.property;
 
-const methodName = node => method(node).name;
+const methodName = node => method(node) && method(node).name;
 
-const methodName2 = node => method2(node).name;
+const methodName2 = node => method2(node) && method2(node).name;
 
 export const argument = node =>
   node.parent.parent.arguments && node.parent.parent.arguments[0];