[BugFix] Fix engine reuse failure caused by empty/null engine instance (#5300)

[zhang-arvin]

Closes #5300

[zhang-arvin]

Base-branch note: the failing checks are not caused by this PR.

All of sql-check, spotless-check, third-party-dependencies-check and build-backend fail on files that this PR does not touch, and they fail identically on the other open PRs from the same branch point. The failures come from master itself:

• sql-check: linkis-dist/package/db/linkis_dml.sql:670 — ERROR 1136: Column count doesn't match value count at row 11 (the Oracle datasource INSERT), introduced by [SECURITY] Block JDBC URL-option injection via DB2 instance field #5450.
• spotless-check: linkis-commons/linkis-common/.../TicketCipher.java format violation, from the DES→AES change in [SECURITY] Upgrade session ticket cipher from DES to AES-256-GCM #5460.
• build-backend / third-party-dependencies-check: linkis-commons/linkis-module/.../ServerConfiguration.scala:80 and SecurityFilter.scala:158 — not found: value CRYPT_KEY_INSECURE / not found: value CommonVars, from [SECURITY] Remove hardcoded default wds.linkis.crypt.key, fail-closed on insecure values #5456.

None of these files appears in this PR's diff, so there is nothing on this PR to fix here; it will go green once master is repaired. This PR stays on the master baseline problem until that lands.

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