[DX] Avoid warning when not needed

[ennioVisco]

Describe the bug

Ideally, this line should be printed only when the output has no imports, but it is printed anytime Options.imports is empty (but users could instead be importing with Options.include or Options.dirs.

Also, the English could be polished a bit and changed here to:
[auto-import] plugin installed but no imports hashave been defined, see https://github.com/antfu/unplugin-auto-import#configurations for configurations'

Validations

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• Read the Contributing Guide.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
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• The provided reproduction is a minimal reproducible of the bug.

[ascott18]

Additionally, the #configurations anchor doesn't exist - it should be #configuration (no s)