fix: set ED to max value for unsupported assets

[enthusiastmartin]

Instead of 0, set MAX value for unsupported assets.

[codecov]

Codecov Report

Merging #177 (f1e5923) into master (6140810) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #177   +/-   ##
=======================================
  Coverage   85.38%   85.38%           
=======================================
  Files          21       21           
  Lines        2108     2108           
=======================================
  Hits         1800     1800           
  Misses        308      308           

Impacted Files	Coverage Δ
pallets/asset-registry/src/lib.rs	91.07% <100.00%> (ø)

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