Fix sanitizeScriptTags when content is null

[G-Ambatte]

This PR resolves #3315.

This PR modifies the sanitizeScriptTags function to properly handle the null case, which will preventing website crashing to a white screen when the content window is empty.

[calculuschild]

Just noting here for future reference: The error is caused by the addition of this line:

homebrewery/client/homebrew/brewRenderer/brewRenderer.jsx

Line 155 in 4feeaee

renderedPages[props.currentEditorPage] = renderPage(rawPages[props.currentEditorPage], props.currentEditorPage);

This line is to make sure when rendering pages, always render the currently-edited page first (which may not be in view on the brewRenderer preview). This ensures that any changes that have cross-page effects (e.g. variables or links) will be parsed first, so the latest values will propagate correctly to the pages in view.

Unfortunately, updating the "current editor page" happens slightly out of sync here:

Codemirror sees a change -> sends this change up to the top at handleTextChange() in editPage.jsx (or newPage or homePage) and gets the currently edited page from editor.jsx -> propagates new text down to editor.jsx and brewRenderer.jsx.

As you can see, editor.jsx is being asked to calculate the latest cursor position before it it has received the latest text.

homebrewery/client/homebrew/pages/editPage/editPage.jsx

Line 116 in 4feeaee

currentEditorPage : this.refs.editor.getCurrentPage() - 1 //Offset index since Marked starts pages at 0

So while this PR does fix the crashing (I will merge this for now), it might be worth moving getCurrentPage() to come directly from Codemirror in codeEditor.jsx instead which will be more accurate.