improvement: Don't allow empty sbtScript setting #13729

Job	Run time
windows-latest jdk-17 unit tests 1 / 2	24m 44s
Gradle MacOS integration	14m 54s
windows-latest jdk-17 unit tests 2 / 2	24m 7s
Sbt integration	47m 33s
macOS-12 jdk-17 unit tests 1 / 2	36m 19s
Maven integration	8m 8s
macOS-12 jdk-17 unit tests 2 / 2	42m 47s
Gradle integration	8m 30s
ubuntu-latest jdk-17 unit tests 1 / 2	18m 41s
ubuntu-latest jdk-17 unit tests 2 / 2	16m 35s
Scala CLI integration	13m 22s
Mill integration	39m 46s
Bazel integration	10m 57s
LSP integration tests	34m 51s
Scala cross tests	36m 4s
Formatting	43s
Scalafix and docs	5m 13s
Sbt-metals/scripted jdk11	2m 38s
Scala javapc tests	1m 48s
Scala 2.11 cross tests	4m 6s
MiMa	49s
	6h 32m 35s

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