fix(completion): fix static analysis shellcheck errors

[leagris]

In ./contrib/completion/bash/sdk line 36:
			candidates=${SDKMAN_CANDIDATES[@]}
                        ^--------^ SC2178 (warning): Variable was used as an array but is now assigned a string.
                                   ^---------------------^ SC2124 (warning): Assigning an array to a string! Assign as array, or use * instead of @ to concatenate.

In ./contrib/completion/bash/sdk line 52:
	COMPREPLY=($(compgen -W "${candidates[*]}" -- "$current_word"))
                   ^-- SC2207 (warning): Prefer mapfile or read -a to split command output (or quote to avoid splitting).

In ./contrib/completion/bash/sdk line 79:
	COMPREPLY=($(compgen -W "${candidates[*]}" -- "$candidate_version"))
                   ^-- SC2207 (warning): Prefer mapfile or read -a to split command output (or quote to avoid splitting).

For more information:
https://www.shellcheck.net/wiki/SC2124 -- Assigning an array to a string! A...
https://www.shellcheck.net/wiki/SC2178 -- Variable was used as an array but...
https://www.shellcheck.net/wiki/SC2207 -- Prefer mapfile or read -a to spli...

[helpermethod]

Hi @leagris,

I've added a few comments to your proposed changes. While these are technically correct, we highly value backwards compatibility, which some of these changes might break.