feature: add defaults for SMTP user and password

[hsiliev]

Allow start of Wildfly without SMTP user and password environment variables

Solves PZ-4748

[github-actions]

unit-test-results

 79 files  ±0  110 suites  ±0   1m 31s ⏱️ ±0s
381 tests ±0  380 ✅ ±0  1 💤 ±0  0 ❌ ±0 
475 runs  ±0  473 ✅ ±0  2 💤 ±0  0 ❌ ±0 

Results for commit 35cea5a. ± Comparison against base commit e87d02f.

♻️ This comment has been updated with latest results.

[github-actions]

integration-test-results

 28 files  ±0   28 suites  ±0   35s ⏱️ ±0s
107 tests ±0  107 ✅ ±0  0 💤 ±0  0 ❌ ±0 
120 runs  ±0  120 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 35cea5a. ± Comparison against base commit e87d02f.

♻️ This comment has been updated with latest results.

[codecov]

Codecov Report

Attention: Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.

Project coverage is 54.76%. Comparing base (e87d02f) to head (35cea5a).
Report is 9 commits behind head on main.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/main/kotlin/net/atos/zac/mail/MailService.kt	57.14%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##               main    #2535    +/-   ##
==========================================
  Coverage     54.76%   54.76%            
- Complexity     3366     3367     +1     
==========================================
  Files          1004     1004            
  Lines         22761    22768     +7     
  Branches       1785     1751    -34     
==========================================
+ Hits          12465    12469     +4     
- Misses         9382     9503   +121     
+ Partials        914      796   -118     

Flag	Coverage Δ
integrationtests	50.04% <42.85%> (-0.01%)	⬇️
unittests	34.84% <57.14%> (+<0.01%)	⬆️

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