…`()` is deprecated

Scala 2.13 deprecates (with PR scala/scala#8833) the old behaviour of Scala that zero-parameter methods could be called with either one or zero pairs of parenthesis - ie if you have a method `def foo()` you could call it as `foo()` or just `foo`. With Scala 3, you have to match the number of brackets *used in the method declaration* when you call it - so you'd _have_ to use `foo()` or you'd get an error like this:

```
[error] ~/code/presence-indicator/app/actor/OpenSocketActor.scala:79:7: Auto-application to `()` is deprecated. Supply the empty argument list `()` explicitly to invoke method sender,
[error] or remove the empty argument list from its definition (Java-defined methods are exempt).
[error] In Scala 3, an unapplied method like this will be eta-expanded into a function. [quickfixable]
[error]       sender ! Map(serverId -> (LiveActors(connectionPing, subscription)))
[error]       ^
```

Java methods are exempt from this restriction - you can call either `hashCode()` (which, in Java, is how the method _has_ to be defined, with empty brackets) or just `hashCode` (which is how that method would have been declared if it was declared in Scala, in Scala methods with no side-effects should be declared without brackets: https://docs.scala-lang.org/style/method-invocation.html#arity-0).

## Automatically fixing this code issue

There are two possible ways of automating this code fix - in this small project, they both produce the same code changes:

### Fixing if the project is already on Scala 2.13 - use `-quickfix` in `scalac`

You can use the `-quickfix` support added to Scala 2.13.12 with scala/scala#10482:

Add either of these to the `scalacOptions` in `build.sbt`:

* `"-quickfix:any"`  ...to apply *all* available quick fixes
* `"-quickfix:msg=Auto-application"`  ...to apply quick fixes where the message contains "Auto-application"

Then run `compile` on the sbt console - the first compile will still fail, but it will subtly change the error message to say `[rewritten by -quickfix]` - your files have been edited to receive the fix:

```
[error] /Users/Roberto_Tyley/code/presence-indicator/app/actor/OpenSocketActor.scala:79:7: [rewritten by -quickfix] Auto-application to `()` is deprecated. Supply the empty argument list `()` explicitly to invoke method sender,
[error] or remove the empty argument list from its definition (Java-defined methods are exempt).
[error] In Scala 3, an unapplied method like this will be eta-expanded into a function.
[error]       sender ! Map(serverId -> (LiveActors(connectionPing, subscription)))
[error]       ^
```

...run `compile` a second time, and compiler will be much happier.

Examples of other PRs using `-quickfix` to fix this code issue:

* guardian/ophan#5719

### Fixing while still on Scala 2.12 - use Scalafix

Fixing this everywhere in a project can be tedious, but thankfully there is a `ExplicitNonNullaryApply` Scalafix rule to fix this in the https://github.com/lightbend-labs/scala-rewrites project.

The Scalafix rule needs to be run while the project is still on Scala 2.12, not Scala 2.13 (otherwise sbt will say: "Error downloading ch.epfl.scala:sbt-scalafix;sbtVersion=1.0;scalaVersion=2.13:0.13.0").

Once the Scalafix plugin is made available to sbt (by adding `addSbtPlugin("ch.epfl.scala" % "sbt-scalafix" % "0.13.0")`
to either `project/plugins.sbt` or `~/.sbt/1.0/plugins.sbt`), you can run these commands on the sbt prompt to automatically generate the changes in this PR:

```
scalafixEnable
scalafixAll dependency:fix.scala213.ExplicitNonNullaryApply@org.scala-lang:scala-rewrites:0.1.5
```

Examples of other PRs using Scalafix to fix this code issue:

* guardian/mobile-apps-api#2728
* guardian/presence-indicator#196

See also:

* scalacenter/scalafix#204
* lightbend-labs/scala-rewrites#14

…-application

Deprecate auto-application of (non-Java-defined) methods with a single empty parameter list

…-application

Deprecate auto-application of (non-Java-defined) methods with a single empty parameter list