Never take apply as loosely nullary

[som-snytt]

Fixes #21351

The example in the doc of bad parens is isWhole(), but no apply method is like that.

Fixes #22061

A class C(implicit c: Int = 42) has a synthetic leading parameter list, but the default arg does not. When constructing a default arg expression, it's necessary to check whether the leading parens are applied.

[som-snytt]

The only usage without parens:

[error] -- [E100] Syntax Error: /home/runner/work/scala3/scala3/community-build/community-projects/munit/tests/jvm/src/test/scala/munit/AsyncFixtureOrderSuite.scala:7:36 
[error] 7 |  val latch: Promise[Unit] = Promise[Unit]
[error]   |                             ^^^^^^^^^^^^^
[error]   |            method apply in object Promise must be called with () argument

Oh by coincidence, also Promise:

[error] -- [E100] Syntax Error: /home/runner/work/scala3/scala3/community-build/community-projects/libretto/core/src/main/scala/libretto/impl/FreeScalaFutureRunner.scala:435:35 
[error] 435 |                    val p = Promise[Frontier[One |+| One]]
[error]     |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[error]     |        method apply in object Promise must be called with () argument
[error]     |
[error]     | longer explanation available when compiling with `-explain`
[error] -- [E100] Syntax Error: /home/runner/work/scala3/scala3/community-build/community-projects/libretto/core/src/main/scala/libretto/impl/FreeScalaFutureRunner.scala:450:25 
[error] 450 |          val p = Promise[() => Frontier[One]]
[error]     |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[error]     |        method apply in object Promise must be called with () argument
[error]     |
[error]     | longer explanation available when compiling with `-explain`
[error] two errors found

It looks like the CB libretto is stale. But this commit has many promises with parens, so these two exceptions prove the rule.

[som-snytt]

munit tweak has been upstreamed, but I'm not sure what's the right approach for updating the CB libretto.