Restore bold uncolored font face (#47108)

Commit aa0825d642 accidentally added a semicolon
to the ANSI escape sequence even if the color code was `None` or unknown, breaking the
bold, uncolored font-face.  This PR restores the old behavior.

---------

Co-authored-by: Todd Gamblin <tgamblin@llnl.gov>

lib/spack/llnl/util/tty/color.py

@@ -263,7 +263,9 @@ def match_to_ansi(match):
                 f"Incomplete color format: '{match.group(0)}' in '{match.string}'"
             )
 
-        ansi_code = _escape(f"{styles[style]};{colors.get(color_code, '')}", color, enclose, zsh)
+        color_number = colors.get(color_code, "")
+        semi = ";" if color_number else ""
+        ansi_code = _escape(f"{styles[style]}{semi}{color_number}", color, enclose, zsh)
         if text:
             return f"{ansi_code}{text}{_escape(0, color, enclose, zsh)}"
         else: