Check nullity of webcontents and empty url from getURL()

[darkdh]

#12508 is caused by empty
url which will leads to sending {cancel: true} to callback of
session.webRequest.onBeforeRequest and download process will be
aborted.
Also verified this won't break the issues which
#12256 fixes

fix #12508

Auditors: @bridiver, @diracdeltas, @bsclifton

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed. (Ask a Brave employee to help if you cannot access this document.)

Test Plan:

Reviewer Checklist:

• Request a security/privacy review as needed if one was not already requested.

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[codecov-io]

Codecov Report

Merging #12555 into master will increase coverage by 0.09%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master   #12555      +/-   ##
==========================================
+ Coverage   55.95%   56.05%   +0.09%     
==========================================
  Files         278      278              
  Lines       27029    27065      +36     
  Branches     4373     4381       +8     
==========================================
+ Hits        15125    15172      +47     
+ Misses      11904    11893      -11

Flag	Coverage Δ
#unittest	56.05% <0%> (+0.09%)	⬆️

Impacted Files	Coverage Δ
app/filtering.js	18.18% <0%> (-0.12%)	⬇️
app/browser/reducers/ledgerReducer.js	47.54% <0%> (-0.5%)	⬇️
js/actions/appActions.js	19.11% <0%> (ø)	⬆️
app/browser/api/ledger.js	53.44% <0%> (+1.67%)	⬆️

[bsclifton]

++! 😄

[bsclifton]

master 726ac1c
0.21.x fcecefd
0.20.x f11fa20