How to reproduce:
1. Create a 2D project
2. Observe the Console window

Expected result: no warnings appear
Actual result: a warning appears

Reproducible with: 2020.1.0a22 (4.1.0), 2020.1.0b4 (4.1.1), 2020.2.0a7 (4.2.1)
Not reproducible with: 2018.4.20f1 (2.0.0-preview.3), 2019.3.8f1 (3.2.1), 2020.1.0a21 (4.1.0)
Could not test with: 2017.4 (package was not yet implemented)

Notes:
A warning does not appear when reopening the project
A warning does not appear when updating or downgrading the package version

The following warning appears:
Assembly for Assembly Definition File 'Packages/com.unity.2d.animation/Tests/Editor/Unity.2D.Animation.EditorTests.asmdef' will not be compiled, because it has no scripts associated with it.
UnityEditor.Scripting.ScriptCompilation.EditorCompilationInterface:TickCompilationPipeline(EditorScriptCompilationOptions, BuildTargetGroup, BuildTarget, String[]) (at /Users/builduser/buildslave/unity/build/Editor/Mono/Scripting/ScriptCompilation/EditorCompilationInterface.cs:398)