fix(trie): differentiate []byte(nil) and []byte{} for storage values

[qdm12]

Changes

Differentiate empty storage values []byte{} with no storage value []byte(nil)

• Leaves can have an empty storage value ([]byte{}), but cannot have no storage value
• Branch with value can have an empty storage value ([]byte{}), but cannot have no storage value
• Branch without value cannot have a value at all ([]byte(nil))

Side fix: do not further read from scale decoder buffer when the slice length is 0, to avoid returning an error (io.EOF)

Tests

go test ./lib/trie/... ./internal/trie/... ./dot/state

Issues

• Solves CRITICAL: Bitfields and heads must be included every block #2937 and closes fixing bitfields and heads must be included every block #2955
• Fix bug introduced in fix(trie): disallow empty byte slice node values #2927 and other misconceptions from original code

Primary Reviewer

@kishansagathiya

[codecov]

Codecov Report

Merging #2964 (a3d2fe3) into qdm12/trie/nodevaluename (9598892) will increase coverage by 0.11%.
The diff coverage is 76.92%.

Additional details and impacted files

@@                     Coverage Diff                      @@
##           qdm12/trie/nodevaluename    #2964      +/-   ##
============================================================
+ Coverage                     63.26%   63.37%   +0.11%     
============================================================
  Files                           218      218              
  Lines                         27505    27503       -2     
============================================================
+ Hits                          17400    17430      +30     
+ Misses                         8503     8469      -34     
- Partials                       1602     1604       +2