backupformat: strip prefixes w/ empty replacement

[jzelinskie]

Description

This adjusts zed backup <cmd> --prefix-replacements flag to support myprefix= to strip off a prefix entirely.

Testing

Easiest way to is to run zed backup parse-schema --prefix-replacements 'myprefix=' on a backup that has a prefix on it.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 42.46%. Comparing base (ec981d0) to head (bf5b805).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/backupformat/rewriter.go	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #620      +/-   ##
==========================================
- Coverage   42.48%   42.46%   -0.02%     
==========================================
  Files          38       38              
  Lines        5030     5032       +2     
==========================================
  Hits         2137     2137              
- Misses       2638     2639       +1     
- Partials      255      256       +1     

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[tstirrat15]

LGTM