Closes gh-20670

Previously, a condition checked the value of "spring.datasource.url" to
determine if an embedded database has to be created as a fallback. When
the value is set with an unresolved placeholder, this fails even if
the DataSource is going to created by another mean ultimately.

This commit makes a more conservative check by only checking the
presence of the property rather than its value.

Closes gh-20438

Closes gh-20669

Closes gh-20571

Closes gh-20658

Closes gh-20657

Closes gh-20656

Closes gh-20655

Closes gh-20654

Closes gh-20653

Closes gh-20652

Closes gh-20651

Closes gh-20650

Closes gh-20649

Closes gh-20648

Closes gh-20647

Closes gh-20646

Closes gh-20645

Closes gh-20644

Closes gh-20659

Closes gh-20643

Closes gh-20642

Closes gh-20641

Closes gh-20640

Closes gh-20639

Closes gh-20638

Closes gh-20637

Closes gh-20636

Closes gh-20635

Closes gh-20634

Closes gh-20633

Closes gh-20632

Closes gh-20631

Closes gh-20569

Closes gh-20624

Closes gh-20378

* pr/20617:
  Polish "Fix @FlywayDataSource with multiple data sources"
  Fix @FlywayDataSource with multiple data sources

Closes gh-20617

See gh-20617