Make IHostEnvironment.ApplicationName non-nullable

[eerhardt]

It is not intended that normal execution has a null ApplicationName, so don't make callers need to check for null. When an ApplicationName is not available, it is set to string.Empty.

Fix#68512

[ghost]

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Issue Details

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It is not intended that normal execution has a null ApplicationName, so don't make callers need to check for null. When an ApplicationName is not available, it is set to string.Empty.

Fix #68512

Author:	eerhardt
Assignees:	-
Labels:	new-api-needs-documentation, area-Extensions-Hosting
Milestone:	-

[eerhardt]

@halter73 - any concern with using string.Empty instead of <Unknown> here?

[halter73]

I think using string.Empty instead of something like <Unknown> is reasonable since hosting assumes that a non-empty app name is the name of the assembly containing the UserSecretsIdAttribute. It's easier to special case string.Empty than <Unknown>, and still communicates that a good default app name couldn't be determined.