RFR: 8349959: Test CR6740048.java passes unexpectedly missing CR6740048.xsd

[SendaoYan]

Hi all,

Test test/jaxp/javax/xml/jaxp/unittest/validation/CR6740048.java run passes unexpectedly when missing the depentdent file test/jaxp/javax/xml/jaxp/unittest/validation/CR6740048.xsd. This PR add a NULL check after call `getResourceAsStream("CR6740048.xsd")`.

This PR do not change the original test logic but make test more robustness. Change has been verified locally, test-fix only, no risk.

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Commit messages:
 - 8349959: Test CR6740048.java passes unexpectedly missing CR6740048.xsd

Changes: https://git.openjdk.org/jdk/pull/23627/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23627&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349959
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/23627.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23627/head:pull/23627

PR: https://git.openjdk.org/jdk/pull/23627