fix npe while checking storage uri scheme

Author: sureshanaparti

plugins/hypervisors/kvm/src/main/java/com/cloud/hypervisor/kvm/storage/KVMStoragePoolManager.java

@@ -296,7 +296,7 @@ public KVMStoragePool getStoragePoolByURI(String uri) {
         String uuid = null;
         String sourceHost = "";
         StoragePoolType protocol = null;
-        final String scheme = storageUri.getScheme().toLowerCase();
+        final String scheme = (storageUri.getScheme() != null) ? storageUri.getScheme().toLowerCase() : "";
         List<String> acceptedSchemes = List.of("nfs", "networkfilesystem", "filesystem");
         if (acceptedSchemes.contains(scheme)) {
             sourcePath = storageUri.getPath();