better error message for empty expression

[Roger-luo]

julia> using MLStyle

julia> @match 1 begin
       end
ERROR: ArgumentError: empty match expression
Stacktrace:
 [1] top-level scope
   @ REPL[2]:1

[codecov]

Codecov Report

Base: 62.64% // Head: 62.68% // Increases project coverage by +0.04% 🎉

Coverage data is based on head (202b23f) compared to base (702a73b).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #150      +/-   ##
==========================================
+ Coverage   62.64%   62.68%   +0.04%     
==========================================
  Files          26       26              
  Lines        1582     1584       +2     
==========================================
+ Hits          991      993       +2     
  Misses        591      591              

Impacted Files	Coverage Δ
src/AbstractPatterns/impl/RedyFlavoured.jl	61.56% <ø> (ø)
src/MatchImpl.jl	72.69% <100.00%> (+0.19%)	⬆️

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