need protection against data.table joins with empty data.tables

[achubaty]

after having fixed several join issues related to duplicate or missing rows (4dd9087), i am still seeing warnings from other places suggesting we are joining with an empty DT in a few more places:

Warning messages:
1: In as.data.table.list(x, keep.rownames = keep.rownames, check.names = check.names,  :
  Item 1 has 0 rows but longest item has 1; filled with NA
2: In as.data.table.list(x, keep.rownames = keep.rownames, check.names = check.names,  :
  Item 2 has 0 rows but longest item has 1; filled with NA
3: In as.data.table.list(x, keep.rownames = keep.rownames, check.names = check.names,  :
  Item 1 has 0 rows but longest item has 1; filled with NA
4: In as.data.table.list(x, keep.rownames = keep.rownames, check.names = check.names,  :
  Item 2 has 0 rows but longest item has 1; filled with NA
5: In as.data.table.list(x, keep.rownames = keep.rownames, check.names = check.names,  :
  Item 1 has 0 rows but longest item has 1; filled with NA
6: In as.data.table.list(x, keep.rownames = keep.rownames, check.names = check.names,  :
  Item 2 has 0 rows but longest item has 1; filled with NA