ci: fix base commit fallback for check-whitespace and check-style

The check-whitespace and check-style CI scripts require a base commit.
In GitLab CI, the base commit can be provided by several different
predefined CI variables depending on the type of pipeline being
performed.

In 30c4f7e350 (check-whitespace: detect if no base_commit is provided,
2024-07-23), the GitLab check-whitespace CI job was modified to support
CI_MERGE_REQUEST_DIFF_BASE_SHA as a fallback base commit if
CI_MERGE_REQUEST_TARGET_BRANCH_SHA was not provided. The same fallback
strategy was also implemented for the GitLab check-style CI job in
bce7e52d4e (ci: run style check on GitHub and GitLab, 2024-07-23).

The base commit fallback is implemented using shell parameter expansion
where, if the first variable is unset, the second variable is used as
fallback. In GitLab CI, these variables can be set but null. This has
the unintended effect of selecting an empty first variable which results
in CI jobs providing an invalid base commit and failing.

Fix the issue by defaulting to the fallback variable if the first is
unset or null.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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