Steps to reproduce:

1) Open Unity and create a new project
2) In Build Settings, switch platform to Android
3) Select Build System as Gradle and enable Export Project
4) Click on Export button
Observe dialog window popping up which says "Release builds must be signed when using Gradle! Please select a keystore and a signing key!"

Expected result: Unity should not require to have a keystore when exporting project with Gradle
Actual result: Unity requires to have a keystore when exporting project with Gradle (check attached image 'bug.png')

Reproduced with:
5.6.1p1, 5.6.1p3, 2017.1.0b9, 2017.2.0a3

Not reproduced with:
5.6.0b10, 5.6.1f1

Regression since:
5.6.1p1
Note that only release builds with Gradle should require to have a keystore(Build || Build and Run), but not exporting projects

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Fixed in: 5.6.3p2, 2017.1.0p5, 2017.2.0b9