memory alloc: Accomodate zero buffer size estimate v2 (#777)

* memory alloc: Accomodate zero buffer size estimate v2

* code-review feedback

Co-authored-by: Paul Hoffman <mojaveazure@users.noreply.github.com>

---------

Co-authored-by: Paul Hoffman <mojaveazure@users.noreply.github.com>

R/TileDBArray.R

@@ -853,9 +853,16 @@ setMethod("[", "tiledb_array",
                          MoreArgs=list(qryptr=qryptr), SIMPLIFY=TRUE)
       ## ensure > 0 for correct handling of zero-length outputs, ensure respecting memory budget
       spdl::debug("['['] result of size estimates is {}", paste(ressizes, collapse=","))
-      resrv <- max(1, min(memory_budget/8, ressizes[ressizes > 0]))
+      idx <- ressizes > 0
+      ressizes <- if (any(idx)) {
+        ressizes[idx] 
+      } else {
+        0
+      }
+      resrv <- max(1, min(memory_budget/8, ressizes))
       spdl::debug("['['] overall estimate {} rows", resrv)
 
+
       ## allocate and set buffers
       if (!use_arrow) {
           getBuffer <- function(name, type, varnum, nullable, resrv, qryptr, arrptr) {