fix(manager/nuget): return null for no deps

[viceice]

Changes

Align with other manager, which returns null if no deps are found in file.
I found a lot empty sections on my dependency dashboard.

Context

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[rarkins]

This PR makes sense now, but later it may make more sense for managers to return package files with no deps in order for it to be more transparent/obvious to the user - we need to work out when to suppress them, e.g. in dashboard

[viceice]

This PR makes sense now, but later it may make more sense for managers to return package files with no deps in order for it to be more transparent/obvious to the user - we need to work out when to suppress them, e.g. in dashboard

Currently it makes viewing logs hard when searching for problems, because i've hundreds of matching files for nuget, but only a few with real dependencies. The others are simply support files and it would be hard to filter them accordingly.

[renovate-release]

🎉 This PR is included in version 35.90.0 🎉

The release is available on:

• GitHub release
• 35.90.0

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