;fix:hledger-bar: really fix the error when NO_COLOR is not defined [#2159]

Also, it's now more compliant with the no-color.org spec:

  Command-line software which adds ANSI color to its output by default
  should check for a NO_COLOR environment variable that, when present
  and not an empty string (regardless of its value), prevents the
  addition of ANSI color.

so one can now temporarily override $NO_COLOR=1 in the environment by
setting it empty: NO_COLOR= hledger ...

bin/hledger-bar

@@ -78,7 +78,7 @@ poschar="+"
 
 # https://en.wikipedia.org/wiki/ANSI_escape_code#8-bit and 24-bit colors.
 # Disable if stdout is not a terminal or NO_COLOR is defined.
-if [[ ! -t 1 || -v  ]]; then
+if [[ ! -t 1 || $NO_COLOR ]]; then
     red=''
     green=''
     nocol=''