Don't print colors when $TERM is 'dumb' or unset (fixes #2260)

CHANGELOG.md

@@ -6,6 +6,7 @@
 ### Fixed
 - SC2102 about repetitions in ranges no longer triggers on [[ -v arr[xx] ]]
 - SC2290: Warn about misused = in declare & co, which were not caught by SC2270+
+- The flag --color=auto no longer outputs color when TERM is "dumb" or unset
 
 ### Changed
 - SC2048: Warning about $\* now also applies to ${array[\*]}