Empty argument list is not shown in scaladoc in some cases

fixes #13148

scaladoc-testcases/src/tests/emptyparens.scala

@@ -0,0 +1,27 @@
+package tests.emptyparens
+
+class C {
+  def f1()(implicit i: Int) = i
+
+  def f2()(using i: Int) = i
+
+  def f3(s: String)(implicit i: Int) = i
+
+  def f4(s: String)(using i: Int) = i
+
+  def f5()()(using i: Int) = i
+
+  def f6() = 1
+
+  def f7()() = 2
+
+  def f8(i: Int)() = 1
+}
+
+class C1()(implicit i: Int)
+
+class C2()(using i: Int)
+
+class C3()()
+
+class C4()(i: Int)

scaladoc/src/dotty/tools/scaladoc/translators/ScalaSignatureUtils.scala

@@ -28,10 +28,11 @@ trait SignatureBuilder extends ScalaSignatureUtils {
       prefix: String = "",
       suffix: String = "",
       separator: String = ", ",
+      forcePrefixAndSuffix: Boolean = false
     )(
       elemOp: (SignatureBuilder, E) => SignatureBuilder
     ): SignatureBuilder = elements match {
-      case Nil => this
+      case Nil => if forcePrefixAndSuffix then this.text(prefix + suffix) else this
       case head :: tail =>
         tail.foldLeft(elemOp(text(prefix), head))((b, e) => elemOp(b.text(separator), e)).text(suffix)
     }
@@ -85,7 +86,7 @@ trait SignatureBuilder extends ScalaSignatureUtils {
       if params.isEmpty then this.text("")
       else if params.size == 1 && params(0).parameters == Nil then this.text("()")
       else this.list(params, separator = ""){ (bld, pList) =>
-        bld.list(pList.parameters, s"(${pList.modifiers}", ")"){ (bld, p) =>
+        bld.list(pList.parameters, s"(${pList.modifiers}", ")", forcePrefixAndSuffix = true){ (bld, p) =>
             val annotationsAndModifiers = bld.annotationsInline(p)
               .text(p.modifiers)
             val name = p.name.fold(annotationsAndModifiers)(annotationsAndModifiers.memberName(_, p.dri).text(": "))