[FG6] `reobfJar` fails when Mixin writes an empty tsrg file

[SquidDev]

Sometimes it's possible to have Mixin configured on a project but have it write an empty xxx-mappings.tsrg file. In this case, the reobfJar task will fail due to an out-of-bounds in srgutils:

Caused by: java.lang.IndexOutOfBoundsException: Index 0 out of bounds for length 0
        at java.base/jdk.internal.util.Preconditions.outOfBounds(Preconditions.java:64)
        at java.base/jdk.internal.util.Preconditions.outOfBoundsCheckIndex(Preconditions.java:70)
        at java.base/jdk.internal.util.Preconditions.checkIndex(Preconditions.java:266)
        at net.minecraftforge.srgutils.InternalUtils.loadNamed(InternalUtils.java:39)
        at net.minecraftforge.srgutils.InternalUtils.load(InternalUtils.java:28)
        at net.minecraftforge.srgutils.IMappingFile.load(IMappingFile.java:27)
        at net.minecraftforge.srgutils.IMappingFile.load(IMappingFile.java:22)
        at net.minecraftforge.gradle.userdev.tasks.RenameJarInPlace.apply(RenameJarInPlace.java:66)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

Reproduction steps

• Download the MDK and convert it to use ForgeGradle 6.
• Add the Mixin Gradle plugin, and set up MixinGradle with the following:

  dependencies { annotationProcessor("org.spongepowered:mixin:0.8.5:processor") }
  mixin { add(sourceSets.main, "examplemod.refmap.json") }

  Note no mixin configs are configured.

• Run ./gradlew reobfJar. The build should fail as build/tmp/compileJava/compileJava-mappings.tsrg is empty.

[SquidDev]

Thank you!