[backport] Don't add arguments templates for parameterless method's completion

When asking for completion on a parameterless method we were trying to install
arguments' templates. This operation in not allowed on parameterless methods,
and it caused a ``java.lang.IllegalStateException: must specify at least one
linked position`` during completion.

``explicitParamNames`` is a ``List[List[String]]`` (because of currying), hence
a parameterless method is represented as ``List(List())``. The arguments'
templates where added only if ``explicitParamNames`` is not empty, however a
``List(List())`` contains exactly one element (the empty list), hence it's
never empty!  The fix is really simple, we just need to flatten the
``explicitParamNames`` before checking for emptiness.

Fixes #1001591

backport to _release/3.0.x_

review by @dragos
(cherry picked from commit d08e999c47f4768179d63c99c7c06268fbea55c7)