[Desktop] SpeedReader sometimes returns a blank page for enabled URLs

[AndriusA]

Description

When SpeedReader rule list says that a URL is rewrite-able but the content isn't found with expected selectors a blank page gets rendered

Steps to Reproduce

1. Enable SpeedReader under brave://flags
2. Navigate To e.g. https://edition.cnn.com/travel/article/wild-swimming-saved-my-life-wellness/index.html
3. Enable SpeedReader for the page by clicking on icon next to address bar

Actual result:

Blank page

Expected result:

Default page if rewriting fails

Reproduces how often:

Easily reproduced

Brave version (brave://version info)

Version/Channel Information:

• Can you reproduce this issue with the current release? no
• Can you reproduce this issue with the beta channel?
• Can you reproduce this issue with the dev channel?
• Can you reproduce this issue with the nightly channel? yes

Other Additional Information:

• Does the issue resolve itself when disabling Brave Shields?
• Does the issue resolve itself when disabling Brave Rewards?
• Is the issue reproducible on the latest version of Chrome?

Miscellaneous Information:

[LaurenWags]

Verified passed with

Brave | 1.12.104 Chromium: 84.0.4147.89 (Official Build) dev (64-bit)
-- | --
Revision | 19abfe7bcba9318a0b2a6bc6634a67fc834aa592-refs/branch-heads/4147@{#852}
OS | macOS Version 10.14.6 (Build 18G3020)

• Verified STR from description

---

Verification passed on

Brave	1.12.105 Chromium: 84.0.4147.105 (Official Build) (64-bit)
Revision	a6b12dfad6663f13a7e16e9a42a6a4975374096b-refs/branch-heads/4147@{#943}
OS	Ubuntu 18.04 LTS

• Verified STR from description

---

Verification passed on

Brave | 1.12.105 Chromium: 84.0.4147.105 (Official Build) (64-bit)
-- | --
Revision | a6b12dfad6663f13a7e16e9a42a6a4975374096b-refs/branch-heads/4147@{#943}
OS | Windows 10 OS Version 1903 (Build 18362.959)

• Verified the STR from the description