Internal: Add some @Nullable annotations and fix related compilation warnings.

Added `@Nullable` to:
  - IndicesService.indexService
  - IndexService.shard
  - IndexService.shardInjector

This change doesn't try to do anything smart but just makes sure that a
*MissingException is thrown instead of a NullPointerException when the requested
object doesn't exist.

This change is mostly indentation: it replaces

```java
IndexService indexService = indicesService.indexService(index);
if (indexService != null) {
  // do something
} else {
  throw new IndexMissingException(new Index(index));
}
```

with

```java
IndexService indexService = indicesService.indexServiceSafe(index);
// do something
```