Fix mill visualize with no arguments failing with MatchError

[Copilot]

Running mill visualize with no arguments crashes with an unhelpful MatchError: List() deep in the resolution stack. This adds an early guard to return a clear failure message instead.

Changes

• VisualizeModule.visualize0 (both the current overload and the deprecated bin-compat one): added upfront empty-tasks check that short-circuits with a descriptive Result.Failure before any task resolution occurs

Before:

scala.MatchError: List() (of class scala.collection.immutable.Nil$)
  mill.resolve.Resolve.handleScriptModule$1(Resolve.scala:395)
  ...

After:

error] visualize requires at least one task to visualize, e.g. `mill visualize __.compile`

[alexarchambault]

Don't use return statements

[Copilot]

Don't use return statements

Done in 7b36cb7 — replaced both return statements with if/else expressions.