[Fix] style-prop-object: do not warn with explicit null or undefined var style props.

Per #812 (comment)

Author: ljharb

lib/rules/style-prop-object.js

@@ -22,6 +22,13 @@ module.exports = {
   },
 
   create: function(context) {
+    /**
+     * @param {object} node An Identifier node
+     */
+    function isNonNullaryLiteral(expression) {
+      return expression.type === 'Literal' && expression.value !== null;
+    }
+
     /**
      * @param {object} node A Identifier node
      */
@@ -34,7 +41,7 @@ module.exports = {
         return;
       }
 
-      if (variable.defs[0].node.init.type === 'Literal') {
+      if (isNonNullaryLiteral(variable.defs[0].node.init)) {
         context.report(node, 'Style prop value must be an object');
       }
     }
@@ -54,7 +61,7 @@ module.exports = {
             if (style) {
               if (style.value.type === 'Identifier') {
                 checkIdentifiers(style.value);
-              } else if (style.value.type === 'Literal' && style.value.value !== null) {
+              } else if (isNonNullaryLiteral(style.value)) {
                 context.report(style.value, 'Style prop value must be an object');
               }
             }
@@ -67,10 +74,7 @@ module.exports = {
           return;
         }
 
-        if (
-          node.value.type !== 'JSXExpressionContainer'
-          || (node.value.expression.type === 'Literal' && node.value.expression.value !== null)
-        ) {
+        if (node.value.type !== 'JSXExpressionContainer' || isNonNullaryLiteral(node.value.expression)) {
           context.report(node, 'Style prop value must be an object');
         } else if (node.value.expression.type === 'Identifier') {
           checkIdentifiers(node.value.expression);

tests/lib/rules/style-prop-object.js

@@ -122,6 +122,20 @@ ruleTester.run('style-prop-object', rule, {
       ].join('\n'),
       parserOptions: parserOptions
     },
+    {
+      code: [
+        'let style = null;',
+        '<div style={style}></div>'
+      ].join('\n'),
+      parserOptions: parserOptions
+    },
+    {
+      code: [
+        'let style = undefined;',
+        '<div style={style}></div>'
+      ].join('\n'),
+      parserOptions: parserOptions
+    },
     {
       code: '<div style={undefined}></div>',
       parserOptions: parserOptions