Using Aggregation on an empty data source crashes the server

[Toshbrown]

Calls to /ts//last/5/min for example before a driver has written any data will cause the following crash:

server.exe: main: report_error:Opps: (Invalid_argument "index out of bounds")Raised at file "format.ml" (inlined), line 239, characters 35-52
server.exe: main: Called from file "format.ml", line 465, characters 8-33
server.exe: main: Called from file "format.ml", line 480, characters 6-24
server.exe: main:
server.exe: main: report_error:Opps: ZMQ.ZMQ_exception(0, "Operation cannot be accomplished in current state")Raised at file "map.ml", line 122, characters 10-25
server.exe: main: Called from file "src0/sexp_conv.ml", line 149, characters 10-37
server.exe: main:

[jptmoore]

Should return empty now.