RFR: 8349959: Test CR6740048.java passes unexpectedly missing CR6740048.xsd [v2]

[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.

SendaoYan has updated the pull request incrementally with one additional commit since the last revision:

  Remove the conditional check that verifies its existence

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/23627/files
  - new: https://git.openjdk.org/jdk/pull/23627/files/699c6371..c0cf0d11

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23627&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23627&range=00-01

  Stats: 7 lines in 1 file changed: 0 ins; 4 del; 3 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