fix: Fix output of calculateShards() when all tests are filtered out (#…

…2129)

When all tests were filtered out, it would throw an error `Invalid shard time`.

## Test Plan
> How do we know the code works?
Create a test apk with one test class with an annotation, and `@Ignored`. 
Filter the tests using that annotation.

Console should print out `No tests for ...` instead of crashing with `Invalid shard time`.

## Checklist
Couldn't find existing tests for this class.

test_runner/src/main/kotlin/ftl/args/ArgsHelper.kt

@@ -243,6 +243,11 @@ object ArgsHelper {
             ) // Avoid unnecessary computing if we already know there aren't tests to run.
         }
         val (ignoredTests, testsToExecute) = filteredTests.partition { it.ignored }
+
+        if (testsToExecute.isEmpty()) {
+            return CalculateShardsResult(listOf(), ignoredTests.map { it.testName })
+        }
+
         val shards = if (args.disableSharding) {
             listOf(
                 Chunk(