Fix small bugs

Closes #187 (closed)

Fix 2 bugs:

• Handle empty list of configuration overrides
• Fix error from additional shutdown call during exitprocess(0)

theodolite-quarkus/src/main/kotlin/theodolite/benchmark/Benchmark.kt

@@ -8,6 +8,6 @@ interface Benchmark {
     fun buildDeployment(
         load: LoadDimension,
         res: Resource,
-        configurationOverrides: List<ConfigurationOverride>
+        configurationOverrides: List<ConfigurationOverride?>
     ): BenchmarkDeployment
 }