refact: conditional checks for token secret before fetch

[ChaiWithJai]

Closes 13847

[github-actions]

Ember Asset Size action

As of 21edd00

Files that got Bigger 🚨:

File	raw	gzip
nomad-ui.js	+216 B	+37 B

Files that stayed the same size 🤷‍:

File	raw	gzip
vendor.js	0 B	0 B
nomad-ui.css	0 B	0 B
vendor.css	0 B	0 B

[tgross]

@ChaiWithJai I'm not entirely sure if this fixes the bug described in #13847. Starting without a token (incognito window) I no longer see the requests to /v1/acl/token/self in the agent logs, but when I click on any of the links on the page, including the link to /ui/settings/tokens, the URL bar is updated but the page is not:

I don't see any HTTP requests hit the agent either (or in the Chrome Tools network tab for that matter).

[github-actions]

Ember Test Audit comparison

	main	21edd00	change
passes	1398	1397	-1
failures	2	2	0
flaky	0	1	+1
duration	000ms	000ms	-000ms

[tgross]

Just leaving a note that my issue above is unrelated to this PR and will be fixed by #14144, which @ChaiWithJai and @philrenaud helped me hack on 😀

[github-actions]

Ember Test Audit flaky tests

Ember Test Audit detected these flaky tests on 21edd00:

• Acceptance | exec: navigating to a task adds its name to the route, chooses an allocation, and assigns a default command

[tgross]

LGTM! Visiting the web UI without a token set, I now only see the following logs as expected:

    2022-08-17T13:02:34.613Z [DEBUG] http: request complete: method=GET path=/v1/agent/health?type=client duration="169.429µs"
    2022-08-17T13:02:35.436Z [DEBUG] http: request failed: method=GET path=/v1/acl/policy/anonymous error="ACL policy not found" code=404
    2022-08-17T13:02:35.436Z [DEBUG] http: request complete: method=GET path=/v1/acl/policy/anonymous duration=5.234403ms
    2022-08-17T13:02:35.485Z [DEBUG] http: request failed: method=GET path=/v1/jobs? error="Permission denied" code=403
    2022-08-17T13:02:35.494Z [DEBUG] http: request complete: method=GET path=/v1/jobs? duration=16.637132ms
    2022-08-17T13:02:35.493Z [DEBUG] http: request complete: method=GET path=/v1/namespaces duration=4.510974ms
    2022-08-17T13:02:35.640Z [DEBUG] http: request failed: method=GET path="/v1/jobs?&index=1" error="Permission denied" code=403
    2022-08-17T13:02:35.641Z [DEBUG] http: request complete: method=GET path="/v1/jobs?&index=1" duration="395.122µs"
    2022-08-17T13:02:35.681Z [DEBUG] http: request complete: method=GET path=/v1/namespaces duration="600.458µs"
    2022-08-17T13:02:36.176Z [DEBUG] http: request complete: method=GET path=/v1/agent/health?type=server duration="162.15µs"
    2022-08-17T13:02:44.615Z [DEBUG] http: request complete: method=GET path=/v1/agent/health?type

[github-actions]

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