last_grant_fetch_stamp is 0 if no grants exist when Rewards is enabled

[LaurenWags]

Description

Found while testing #1793

If you enable Brave Rewards when no grants are available, your last_grant_fetch_stamp is set to 0. Opening Rewards page does not change this value.

Steps to Reproduce

Pre-requisite - ensure no grants are available (QA used staging)

1. Clean profile, launch Brave (/Applications/Brave\ Browser.app/Contents/MacOS/Brave\ Browser --rewards-env=stag)
2. Enable Rewards
3. Verify you are not prompted about grants (since none exist).
4. If Rewards page is open, close it. Navigate to a site. Wait minimum amount of time for it to get added to your auto contribute table.
5. Open Rewards page.
6. Open your ledger_state file and locate last_grant_fetch_stamp

Actual result:

last_grant_fetch_stamp has a value of 0

Expected result:

last_grant_fetch_stamp should have a timestamp of when you opened the Rewards page

Reproduces how often:

easily

Brave version (brave://version info)

Brave	0.56.8 Chromium: 70.0.3538.77 (Official Build) (64-bit)
Revision	0f6ce0b0cd63a12cb4eccea3637b1bc9a29148d9-refs/branch-heads/3538@{#1039}
OS	Mac OS X

Reproducible on current release:

• Does it reproduce on brave-browser dev/beta builds?

Website problems only:

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

Additional Information

Reproduced by @kjozwiak

[btlechowski]

Verification passed on

Brave	0.58.11 Chromium: 71.0.3578.80 (Official Build) beta (64-bit)
Revision	2ac50e7249fbd55e6f517a28131605c9fb9fe897-refs/branch-heads/3578@{#860}
OS	Windows 7 x64

Used Test Plan from OP.

Verification passed on Ubuntu 17.10

Brave	0.58.11 Chromium: 71.0.3578.80 (Official Build) beta(64-bit)
Revision	2ac50e7249fbd55e6f517a28131605c9fb9fe897-refs/branch-heads/3578@{#860}
OS	Linux

Used Test Plan from OP.

Verified passed with

Brave	0.58.11 Chromium: 71.0.3578.80 (Official Build) beta(64-bit)
Revision	2ac50e7249fbd55e6f517a28131605c9fb9fe897-refs/branch-heads/3578@{#860}
OS	Mac OS X

• Verified STR from description