fix(prefixStorage): handle missing key

[farnabaz]

key argument is optional in some functions, like (getKeys)

• Add default value for key argument to prevent undefined key (base:undefined)
• Normalize base + key to remove ending : in case of missing key (base:)

[codecov-commenter]

Codecov Report

Merging #18 (933fb2c) into main (15fec29) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #18   +/-   ##
=======================================
  Coverage   89.94%   89.94%           
=======================================
  Files          12       12           
  Lines         398      398           
  Branches       78       79    +1     
=======================================
  Hits          358      358           
  Misses         40       40           

Impacted Files	Coverage Δ
src/utils.ts	90.00% <100.00%> (ø)

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