fix(alert): guard against null alert/dateCreated in email alert template

[wy471x]

Move null check before alert dereferencing in EmailAlertNotifyStrategy to prevent NullPointerException. Add @notblank validation on AlarmContent title and content fields. Add unit tests for both fixes.

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• You have read the contribution guidelines.
• You submit test cases (unit or integration tests) that back your changes.
• Your local test passed ./mvnw clean install -Dmaven.javadoc.skip=true.

Summary of Changes

Bug Fix — EmailAlertNotifyStrategy.java

File: shenyu-alert/src/main/java/org/apache/shenyu/alert/strategy/EmailAlertNotifyStrategy.java

Moved the null check before any dereferencing of alert. Previously, alert.getContent() and
alert.getDateCreated() were called on lines 81-83 while the null guard if (Objects.isNull(alert))
sat dead on line 84. Now both alert == null and dateCreated == null are guarded with safe
fallbacks ("" for content, new Date() for date), matching the pattern already used in
AbstractAlertNotifyHandler.renderContent().

Bean Validation — AlarmContent.java

File: shenyu-common/src/main/java/org/apache/shenyu/common/dto/AlarmContent.java

Added @notblank constraints on title and content fields. The existing ExceptionHandlers already
catches MethodArgumentNotValidException, so invalid requests now get a controlled 400 response
instead of an internal error.

Dependency — shenyu-common/pom.xml

Added jakarta.validation-api (provided scope) since AlarmContent now imports @notblank.

Unit Tests

• AlarmContentTest.java (shenyu-common) — verifies @notblank annotations are present on title and
  content, plus builder/getter/setter correctness.
• EmailAlertNotifyStrategyTest.java (shenyu-alert) — verifies no NPE is thrown when alert is null,
  dateCreated is null, or content is null.

close #6446