src: fix dotenv empty values and indented comments

[christianaurichzm]

parseEnv(), loadEnvFile() and --env-file mishandle empty values and indented comments:

const { parseEnv } = require('node:util');

parseEnv('EMPTY=   \nDATABASE_URL=postgres://localhost');
// { EMPTY: 'DATABASE_URL=postgres://localhost' }

parseEnv('export API_KEY=');      // { 'export API_KEY': '' }
parseEnv('=\nB=2');               // { '': '', B: '2' }
parseEnv('A="x"\n  # C=value');   // { A: 'x', '# C': 'value' }

The empty-value check added in #56983 runs before the key is normalized, and with spaces after = it misses and trim_spaces() eats the newline. Separately, the start of each line is only trimmed at the end of the loop body, which the branches ending in continue skip.

This normalizes the key first, skips only spaces and tabs after =, and moves the line trim into the for increment so it also runs after continue. The new test cases fail on main and pass with this change.

[nodejs-github-bot]

Review requested:

• @nodejs/config

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.46%. Comparing base (3b639be) to head (0434b4d).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66685      +/-   ##
==========================================
- Coverage   92.82%   90.46%   -2.36%     
==========================================
  Files         422      791     +369     
  Lines      194189   277086   +82897     
  Branches    30027    53275   +23248     
==========================================
+ Hits       180249   250674   +70425     
- Misses      13614    16813    +3199     
- Partials      326     9599    +9273     

Files with missing lines	Coverage Δ
src/node_dotenv.cc	86.05% <100.00%> (ø)

... and 501 files with indirect coverage changes

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