EAM API: clarify appinstances get behavior

[gainsley]

What type of PR is this?

• correction

What this PR does / why we need it:

Clarifies the behavior of the GET appinstances API if there are no matching instances to return. Implementors may consider returning 404 in this case, but I believe it is more correct to return 200 with an empty list, because:

• 404 is also returned if the URL path is incorrect, which makes it hard to distinguish between no matches vs invalid URL path
• Users (in particular tests) may intentionally be checking for the absence of an instance. Returning 404 in this case complicates verifying the absence of an instance.

Which issue(s) this PR fixes:

Special notes for reviewers:

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