skeema / skeema / 8209495315

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workspace=docker: fix panic from 8.0 workspace, 5.7 server, utf8mb3 cols

When using workspace=docker and running Skeema from an ARM CPU (e.g. Apple
Silicon Mac, AWS Graviton, etc), and the target database server is running
MySQL 5.7 or older, `skeema diff` and `skeema push` could encounter an
unhandled panic if any column of any table used the legacy 3-byte utf8
character set.

This occurred because MySQL 5.7 (or older) Docker images are not available on
ARM, so MySQL 8 is substituted, with a warning message about this logged.
Recent releases of MySQL 8 refer to 3-byte utf8 as utfmb3 in SHOW CREATE
TABLE and information_schema, whereas older MySQL releases called it utf8.
Skeema's diff logic detects this character set difference, but knows to
disregard it, since these are just two different names for the same character
set. However, Skeema's unsafe-change detection incorrectly sees this as a
change in column character set, without disregarding it. This led to a nil
pointer dereference panic.

This commit fixes the problem by moving the unsafe-diff-handling portion of
the relevant code path into a conditional block which ensures the difference
isn't already being suppressed.

Fixes #226.

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