Fix _getPermittedAccounts type safety

[rekmarks]

PermissionsController._getPermittedAccounts could return an array or null, but it was used in removeAllAccountPermissions as though it could only return an array.

I don't believe it would ever return anything other than an array as things stand, but it's not type safe, and we should fix it. Things with getAccounts in the name tend to only return arrays in our codebase, so I made _getPermittedAccounts only return arrays, and updated calls expecting null to expect an empty array instead.

[metamaskbot]

Builds ready [ce2f5ad]

• builds: chrome, firefox, opera
• bundle viz: background, ui, inpage, contentscript, ui-libs, bg-libs, phishing-detect
• build viz: Build System
• code coverage: Report
• storybook: Storybook
• all artifacts

Page Load Metrics (601 ± 63 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	45	71	60	7	3
		domContentLoaded	368	850	600	132	63
		load	369	851	601	132	63
		domInteractive	367	850	600	132	63

[darkwing]

I very, very much like this change! LGTM!