Fix header checker when copyright regex is None (FreeRTOS#843)

Allow header_checker.py to work as it did before when no
copyright regex is defined.

Author: paulbartell

.github/scripts/common/header_checker.py

@@ -72,7 +72,10 @@ def __init__(
         self.padding = padding
         self.header = header
 
-        self.copyright_regex = re.compile(copyright_regex)
+        if copyright_regex:
+            self.copyright_regex = re.compile(copyright_regex)
+        else:
+            self.copyright_regex = None
 
         # Construct mutated header for assembly files
         self.asm_header = [";" + line for line in header]