Allow Waitall! for an empty input reqs = [] (#458)

* Allow `Waitall!` for an empty input `reqs = []`
* Add a test

Author: ali-ramadhan

src/pointtopoint.jl

@@ -447,6 +447,7 @@ $(_doc_external("MPI_Waitall"))
 """
 function Waitall!(reqs::Vector{Request})
     count = length(reqs)
+    count == 0 && return Status[]
     reqvals = [reqs[i].val for i in 1:count]
     stats = fill(STATUS_EMPTY, count)
     # int MPI_Waitall(int count, MPI_Request array_of_requests[],

test/test_sendrecv.jl

@@ -132,5 +132,7 @@ MPI.Sendrecv!(a, dest_rank, 2,
 
 @test b == [(comm_rank+1) % comm_size, (comm_rank+1) % comm_size, (comm_rank+1) % comm_size]
 
+@test MPI.Waitall!(MPI.Request[]) == MPI.Status[]
+
 MPI.Finalize()
 # @test MPI.Finalized()