tests: shell: Sanitize nft-only/0009-needless-bitwise_0

tests: shell: Sanitize nft-only/0009-needless-bitwise_0

Some versions of awk (gawk-4.2.1-4.el8 in particular) also print the
non-debug ruleset listing's empty lines, causing the diff to fail. Catch
this by exiting upon seeing the first table heading. For the sake of
comparing bytecode, the actual ruleset listing is not interesting,
anyway.

Fixes: 0f7ea0390b336 ("tests/shell: Fix nft-only/0009-needless-bitwise_0")
Signed-off-by: Phil Sutter <phil@nwl.cc>