Allow empty as an argument in a function call

[iamdefinitelyahuman]

What I did

Allow use of empty(Bytes[n]) as an argument within a function call.

Closes #2112

How I did it

• In vyper.parser.parser_utils::pack_arguments, add a branch of logic to handle empty. Because empty implies a length of 0 bytes, the logic here is very simple: add 64 empty bytes from the end of calldata.
• As an optimization, I'm also applying the same logic for literal b"" and "". The overhead of the bytes copier is unnecessary in these situations.
• I've added an optimization where the dynamic pointer is not updated after storing the final dynamic type.

How to verify it

Run the tests. I added some cases.

Cute Animal Picture

[codecov-io]

Codecov Report

Merging #2234 (448a5b6) into master (a222091) will decrease coverage by 1.11%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master    #2234      +/-   ##
==========================================
- Coverage   85.37%   84.25%   -1.12%     
==========================================
  Files          84       84              
  Lines        8552     8494      -58     
  Branches     2066     2069       +3     
==========================================
- Hits         7301     7157     -144     
- Misses        745      831      +86     
  Partials      506      506              

Impacted Files	Coverage Δ
vyper/parser/parser_utils.py	81.02% <91.66%> (-0.39%)	⬇️
vyper/ast/annotation.py	87.87% <0.00%> (-6.82%)	⬇️
vyper/context/types/value/numeric.py	77.50% <0.00%> (-6.65%)	⬇️
vyper/functions/functions.py	83.99% <0.00%> (-5.43%)	⬇️
vyper/parser/expr.py	72.58% <0.00%> (-5.02%)	⬇️
vyper/ast/nodes.py	91.80% <0.00%> (-1.58%)	⬇️
vyper/functions/convert.py	73.64% <0.00%> (-1.36%)	⬇️
vyper/context/types/indexable/sequence.py	82.14% <0.00%> (-0.62%)	⬇️
vyper/functions/signatures.py	65.51% <0.00%> (-0.59%)	⬇️
vyper/context/types/meta/struct.py	80.00% <0.00%> (-0.33%)	⬇️
... and 9 more

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[fubuloubu]

you shut your damn mouth codecov-io bot!