do not allow showing resources blocked if there's none

[cezaraugusto]

fix brave/brave-browser#419
fix brave/brave-browser#429

Test Plan #1: (as described in brave/brave-browser#419)

1. Visit wikipedia.org in a new tab
2. Ensure shields shows badge as 0
3. Open shields, you can click on all shields stats even though it shows 0 but it should not show any resource blocked as there's none.

Test Plan #2: (as described in brave/brave-browser#429)

1. Visit icloud.com in a new tab and change to Block all fingerprinting
2. See blocked count in shields
3. Visit nytimes.com in the same tab
4. Ensure there are no fingerprint blocked items
5. Click on 0 Fingerprint blocked,. should not show any resource blocked as there's none

[yrliou]

I think we should reset resource arrays similar as what we did here, https://github.com/brave/brave-extension/blob/master/app/background/reducers/shieldsPanelReducer.ts#L67-L74, wdyt?
And probably have some unit tests added in

brave-extension/test/app/background/reducers/shieldsPanelReducerTest.ts

Line 31 in eb9b817

describe('ON_BEFORE_NAVIGATION', function () {

.

[bbondy]

re: #42 (review) I think it can be done in a follow up if needed, so merging this.