Fix list warnings

[CryptoPascal31]

Fix several empty list cases reported by the compiler:

lib/ChainwebData/Backfill.hs:33:49: warning: [GHC-63394] [-Wx-partial]
    In the use of ‘head’
    (imported from Prelude, but defined in GHC.Internal.List):
    "This is a partial function, it throws an error on empty lists. Use pattern matching, 'Data.List.uncons' or 'Data.Maybe.listToMaybe' instead. Consider refactoring to use "Data.List.NonEmpty"."
   |
33 |             ranges = map (Low . last &&& High . head) $
   |                                                 ^^^^

lib/ChainwebData/Backfill.hs:66:41: warning: [GHC-63394] [-Wx-partial]
    In the use of ‘head’
    (imported from Prelude, but defined in GHC.Internal.List):
    "This is a partial function, it throws an error on empty lists. Use pattern matching, 'Data.List.uncons' or 'Data.Maybe.listToMaybe' instead. Consider refactoring to use "Data.List.NonEmpty"."
   |
66 |     ranges = map (Low . last &&& High . head) $ groupsOf blockRequestSize [maxi, maxi-1 .. 0]
   |                                         ^^^^



exec/Chainweb/RichList.hs:73:30: warning: [GHC-63394] [-Wx-partial]
    In the use of ‘tail’
    (imported from Prelude, but defined in GHC.Internal.List):
    "This is a partial function, it throws an error on empty lists. Replace it with 'drop' 1, or use pattern matching or 'GHC.Internal.Data.List.uncons' instead. Consider refactoring to use "Data.List.NonEmpty"."
   |
73 |                   . (zip <*> tail)
   |                              ^^^^