spelling: call it null-terminate consistently

With a dash, using two Ls. Also for different forms of the word.

Use NULL in all uppercase if it means a zero pointer.

Follow-up to 307b7543ea

Closes #17489

src/tool_getparam.c

@@ -78,7 +78,7 @@ static ParameterError getstrn(char **str, const char *val,
     return PARAM_NO_MEM;
 
   memcpy(*str, val, len);
-  (*str)[len] = 0; /* null terminate */
+  (*str)[len] = 0; /* null-terminate */
 
   return PARAM_OK;
 }