Steps to reproduce:

1) Open Unity and create a new project
2) Switch Platform to Android
3) In the Player Settings, fill Package Name field
4) In the Build Settings, select Gradle Build System
5) Enable Export Project
6) Click on Export button

Expected result: Error "Release builds must be signed when using Gradle! Please select a keystore and a signing key!" should appear on Release Build if there is no keystore selected
Actual result: There is no error when exporting and keystore is not selected on Release Build

Reproduced with:
5.6.3p4, 2017.1.1p4, 2017.2.0f3, 2017.3.0b3

Regression since:
For 2017.2 version - 2017.2.0b2
For 2017.1 version - 2017.1.0p2
For 5.6 version - 5.6.3p2

Not reproduced with:
5.6.3p1, 2017.1.0p1, 2017.2.0b1

Note: Development Build works as expected, keystore is not required when exporting this project