Fix subscription test failure

[akgarg06]

Describe your changes

• Fixed IntegrationTest_Subscription failing at @BeforeAll with AssertionFailedError: Initial subscription should succeed ==> expected: <0> but was: <1>
• Root cause: cf-subscribe.sh and cf-unsubscribe.sh crashed with a bash unbound variable error before the subscription could be attempted

Fix

Added :- (default-empty) modifier to the indirect expansion:

# Before
consumerSubaccountIdMT="${!SUBACCOUNT_VAR}"

# After
consumerSubaccountIdMT="${!SUBACCOUNT_VAR:-}"

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Checklist before requesting a review

• I follow Java Development Guidelines for SAP
• I have tested the functionality on my cloud environment.
• I have provided sufficient automated/ unit tests for the code.
• I have increased or maintained the test coverage.
• I have ran integration tests on my cloud environment.
• I have validated blackduck portal for any vulnerability after my commit.

Multi-tenancy test cases : https://github.com/cap-java/sdm/actions/runs/36823932306

[PujaDeshmukh17]

LGTM