Don't render Markdown if input or sanitized input is empty

src/core/components/providers/markdown.jsx

@@ -11,6 +11,12 @@ const sanitizeOptions = {
 
 function Markdown({ source }) {
   const sanitized = sanitize(source, sanitizeOptions)
+
+  // sometimes the sanitizer returns "undefined" as a string
+  if(!source || !sanitized || sanitized === "undefined") {
+    return null
+  }
+
   return <Remarkable
     options={{html: true, typographer: true, linkify: true, linkTarget: "_blank"}}
     source={sanitized}

test/bugs/3279-empty-markdown-source.js

@@ -0,0 +1,37 @@
+/* eslint-env mocha */
+import React from "react"
+import expect from "expect"
+import { render } from "enzyme"
+import Markdown from "components/providers/markdown"
+
+describe("UI-3279: Empty Markdown inputs causing bare `undefined` in output", function(){
+  it("should return no text for `null` as source input", function(){
+    let props = {
+      source: null
+    }
+
+    let el = render(<Markdown {...props}/>)
+
+    expect(el.text()).toEqual("")
+  })
+
+  it("should return no text for `undefined` as source input", function(){
+    let props = {
+      source: undefined
+    }
+
+    let el = render(<Markdown {...props}/>)
+
+    expect(el.text()).toEqual("")
+  })
+
+  it("should return no text for empty string as source input", function(){
+    let props = {
+      source: ""
+    }
+
+    let el = render(<Markdown {...props}/>)
+
+    expect(el.text()).toEqual("")
+  })
+})