build.gradle: Fix Android Studio code inspection warnings

Line 18/19: "Method call is ambiguous"
Line 21: "Cannot infer argument types"

These appear to be harmless, as the build still succeeds, but they were bothering me. Apparently,
Android Studio doesn't approve of assigning a blank signingConfig inside the buildType. Assigning it
inside the if-statement (after finding the storeFile environment variable and building the
signingConfig) makes it happy.