on empty peaks: type fix

Author: tungli

src/findpeaks.jl

@@ -25,7 +25,10 @@ Macro specific to findpeaks function."
 macro on_empty_return(peaks, x)
     quote 
         if isempty($(esc(peaks)))
-            return (empty($(esc(x))), empty($(esc(x)), PeakInfo))
+            return (
+                    empty($(esc(x)), Tuple{eltype($(esc(x)))}),
+                    empty($(esc(x)), PeakInfo)
+                   )
         end
     end
 end