[Tests] Fix exception from SealedClassInheritorsProviderForTests

'getValue' throws if the key isn't present.

Modules without .kt-files might be legitimately absent in inheritorsMap
(see org.jetbrains.kotlin.analysis.low.level.api.fir.compiler.based.SealedClassesInheritorsCaclulatorPreAnalysisHandler)

Previously, such tests were not failing because if a module doesn't
contain .kt-files, then it contains only .java files. And there was
another bug in the tests infra that was essentially folding all the
.java-files in one main-module, effectively shadowing the bug in subject
of this commit.

analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/services/LLSealedInheritorsProviderFactoryForTests.kt

@@ -39,7 +39,9 @@ private class SealedClassInheritorsProviderForTests(
             else -> ktModule
         }
 
-        val inheritorsForModuleMap = inheritorsByModule.getValue(relevantModule)
+        // the module might be absent in the map if it doesn't have any .kt-files whatsoever, in which case
+        // there's definitely no sealed inheritors
+        val inheritorsForModuleMap = inheritorsByModule[relevantModule] ?: return emptyList()
         return inheritorsForModuleMap[firClass.classId] ?: firClass.sealedInheritorsAttr?.value ?: emptyList()
     }
 }