improvement: Don't allow empty sbtScript setting #13724

Job	Run time
windows-latest jdk-17 unit tests 1 / 2	23m 15s
windows-latest jdk-17 unit tests 2 / 2	27m 51s
Gradle MacOS integration	14m 14s
macOS-12 jdk-17 unit tests 1 / 2	37m 28s
Sbt integration	47m 49s
macOS-12 jdk-17 unit tests 2 / 2	49m 41s
Maven integration	8m 24s
ubuntu-latest jdk-17 unit tests 1 / 2	19m 25s
Gradle integration	9m 48s
ubuntu-latest jdk-17 unit tests 2 / 2	23m 24s
Scala CLI integration	13m 20s
Mill integration	29m 19s
Bazel integration	10m 50s
LSP integration tests	33m 22s
Scala cross tests	34m 10s
Formatting	36s
Scalafix and docs	5m 1s
Sbt-metals/scripted jdk11	2m 49s
Scala javapc tests	1m 44s
Scala 2.11 cross tests	3m 59s
MiMa	53s
	6h 37m 22s

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