Defaulting to [] in sys.config.src does not work.

Example 1:
in: [{replace_os_vars_tests, [{var1, ${VAR1}}]}].
VAR1=[] awk ... < in > out
out: [{replace_os_vars_tests, [{var1, []}]}].
Works as intended.

Example 2:
in: [{replace_os_vars_tests, [{var1, ${VAR1:-[]}}]}].
awk ... < in > out

awk: run time error: regular expression compile failed (bad class -- [], [^] or [)
[$]{VAR1:-[]}
	FILENAME="-" FNR=1 NR=1

Does not work.

Example 3:
in: [{replace_os_vars_tests, [{var1, ${VAR1:-[]}}]}].
VAR1=[] awk ... < in > out

awk: run time error: regular expression compile failed (bad class -- [], [^] or [)
[$]{VAR1:-[]}
	FILENAME="-" FNR=1 NR=1

Does not work.

Example 4:
in: [{replace_os_vars_tests, [{var1, ${VAR1:-\[\]}}]}].
awk ... < in > out
(endless loop with 100% cpu usage)
Does not work.

Example 5:
in: [{replace_os_vars_tests, [{var1, ${VAR1:-\[\]}}]}].
VAR1=[] awk ... < in > out
(endless loop with 100% cpu usage)
Does not work.

Example 6:
in: [{replace_os_vars_tests, []}]. % fixme: [{var1, ${VAR1:-[]}}]}].
awk ... < in > out

awk: run time error: regular expression compile failed (bad class -- [], [^] or [)
[$]{VAR1:-[]}
	FILENAME="-" FNR=1 NR=1

Does not work, which is notable since it ideally it should be seen as "commented out code" and should not have been regexed. (This is how this was detected in the first place)

These tests were performed with:

awk --version
mawk 1.3.4 20240123