SpecParsingError: only print context if available

Signed-off-by: Gregory Becker <becker33@llnl.gov>

lib/spack/spack/spec_parser.py

@@ -578,6 +578,7 @@ class SpecParsingError(spack.error.SpecSyntaxError):
 
     def __init__(self, message, token, text):
         message += f"\n{text}"
+        if token:
             underline = f"\n{' '*token.start}{'^'*(token.end - token.start)}"
             message += color.colorize(f"@*r{{{underline}}}")
         super().__init__(message)