Revert "Update setPreview to properly display falsy values in preview tooltip"

[darkwing]

Reverts #8027

While reviewing the latest release (https://phabricator.services.mozilla.com/D21608), the following caused the debugger to go blank:

1. Go to https://davidwalsh.name
2. Add breakpoint to line 1 of main.js
3. Refresh browser, breakpoint will hit
4. Hover over "hash"
5. Debugger bricks

The following error is shown:

console.error: (new TypeError("value is null", "resource://devtools/client/debugger/new/src/components/Editor/Preview/index.js", 149))
JavaScript error: resource://devtools/client/debugger/new/src/components/Editor/Preview/index.js, line 149: TypeError: value is null

After removing this commit, the debugger no longer bricks.

We need to further investigate why but in the mean time we should revert this commit.

cc @ravefalcon92

[anthonyx]

I'll take a look into what's causing this and look for an alternative solution. Thanks for finding the issue and letting me know!

[darkwing]

Per the instructions above, hovering over the n before ().hash returns the following object from await client.evaluateInFrame:

{
  errorMessageName: "JSMSG_NOT_DEFINED"
  exception: {…}
​
  actor: "server1.conn1.child1/obj285"
​
  class: "Error"
​
  extensible: true
​
  frozen: false
​
  ownPropertyLength: 4
​
  preview: {…}
​
  sealed: false
​
  type: "object"
​
  <prototype>: {…}
  exceptionDocURL: "https://developer.mozilla.org/docs/Web/JavaScript/Reference/Errors/Not_defined?utm_source=mozilla&utm_medium=firefox-console-errors&utm_campaign=default"
  exceptionMessage: "ReferenceError: n is not defined"
  frame: {…}
​
  column: 1
​
  line: 1
​
  source: "debugger eval code"
​
  <prototype>: {…}
  helperResult: null
  input: "n"
  notes: null
  result: {…}
​
  type: "undefined"
​
<prototype>: {…}
resultID: 1551742481317
timestamp: 1551742481317

The result for hash, which bricks the debugger, is:

{ result: null }