Disable boot jars package check if no boot jars have been configured

Some build configurations do not configure boot jars. In those cases
the boot jars package check fails so should be skipped.

Bug: 177892522
Test: m nothing
      lunch fvp_mini && m
Change-Id: I5498a8f5cdfb2fabf5223e94dfcee0e853059d01
diff --git a/java/boot_jars.go b/java/boot_jars.go
index 86ebe36..5d40ec3 100644
--- a/java/boot_jars.go
+++ b/java/boot_jars.go
@@ -31,13 +31,18 @@
 // buildRuleForBootJarsPackageCheck generates the build rule to perform the boot jars package
 // check.
 func buildRuleForBootJarsPackageCheck(ctx android.ModuleContext, bootDexJarByModule bootDexJarByModule) {
+	bootDexJars := bootDexJarByModule.bootDexJarsWithoutCoverage()
+	if len(bootDexJars) == 0 {
+		return
+	}
+
 	timestamp := android.PathForOutput(ctx, "boot-jars-package-check/stamp")
 
 	rule := android.NewRuleBuilder(pctx, ctx)
 	rule.Command().BuiltTool("check_boot_jars").
 		Input(ctx.Config().HostToolPath(ctx, "dexdump")).
 		Input(android.PathForSource(ctx, "build/soong/scripts/check_boot_jars/package_allowed_list.txt")).
-		Inputs(bootDexJarByModule.bootDexJarsWithoutCoverage()).
+		Inputs(bootDexJars).
 		Text("&& touch").Output(timestamp)
 	rule.Build("boot_jars_package_check", "check boot jar packages")