Add support for operator-only conditions

[Levdbas]

Add Support for Operator-Only Conditions

This pull request recreates the functionality proposed in the closed PR #187.

Thank you to the original author, @nlemoine.

Changes

• Allow field conditional rules to omit value for single-operator conditions such as !=empty.
• Support the same operator-only form when chaining fluent and() and or() calls.
• Omit the value key entirely when no value is supplied, matching ACF's expected configuration format.
• Keep location rules value-required and throw a clear InvalidArgumentException when one is omitted.

Validation

• ConditionalBuilderTest passes, including operator-only constructor and fluent-chain coverage.
• LocationBuilderTest passes, including required-value coverage.
• PHP syntax and whitespace checks pass.
• The full suite has one unrelated PHPUnit 10 compatibility error in ParentDelegationBuilderTest: MockBuilder::setMethods() has been removed.

[Levdbas]

Waiting for #202 to be merged in