entry on controlled input with `value` null

cookbook/cb-08-controlled-input-null-value tip.md

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+---
+id: controlled-input-null-value-tip
+title: Value of null for controlled input
+layout: docs
+permalink: controlled-input-null-value-tip.html
+---
+
+With a controlled input component, specifying a `value` prevents the user from changing the input unless you desire so (more info [here](forms.html)).
+
+You might have ran into a problem where you specified a `value` but the input can still be changed. In this case, you might have accidentally set your `value` to `undefined` or `null`. The snippet below shows this phenomenon; after a second, the text be edited.
+
+```js
+/** @jsx React.DOM */
+
+React.renderComponent(<input value="hi" />, mountNode);
+
+setTimeout(function() {
+  React.renderComponent(<input value={null} />, mountNode);
+}, 2000);
+```

cookbook/cb-08-controlled-input-null-value.md

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+---
+id: controlled-input-null-value
+title: Value of null for controlled input
+layout: docs
+permalink: controlled-input-null-value.html
+---
+
+### Problem
+You specified a `value` parameter for your form input, but the input value can still be modified, contrary to [what you'd expect](forms.html).
+
+### Solution
+You might have accidentally set your `value` to `undefined` or `null`. The snippet below shows this phenomenon; after a second, the text be edited.
+
+```js
+/** @jsx React.DOM */
+
+React.renderComponent(<input value="hi" />, mountNode);
+
+setTimeout(function() {
+  React.renderComponent(<input value={null} />, mountNode);
+}, 2000);
+```