NullPointerException inside LibraryContentProvider breaks the compile…

…r's workflow (#1567)

* Fixing obscure NullPointerException when loading and compiling dependent libraries of a library. The LibraryContentProvider must return null if the library isn't found (as opposed to triggering an NPE) to allow next providers to continue the process.

* Spotless check

workflow/src/main/java/com/google/android/fhir/workflow/FhirEngineLibraryContentProvider.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2021 Google LLC
+ * Copyright 2022 Google LLC
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -26,7 +26,7 @@ class FhirEngineLibraryContentProvider(adapterFactory: AdapterFactory) :
   BaseFhirLibraryContentProvider(adapterFactory) {
   val libs = mutableMapOf<String, Library>()
 
-  override fun getLibrary(libraryIdentifier: VersionedIdentifier): IBaseResource {
-    return libs[libraryIdentifier.id]!!
+  override fun getLibrary(libraryIdentifier: VersionedIdentifier): IBaseResource? {
+    return libs[libraryIdentifier.id]
   }
 }