parse-util: allow parse_boolean() to take a NULL argument

It's pretty useful to allow parse_boolean() to take a NULL argument and
return an error in that case, rather than abort. i.e. making this a
runtime rather than programming error allows us to shorten code
elsewhere.

src/basic/parse-util.c

@@ -20,7 +20,8 @@
 #include "string-util.h"
 
 int parse_boolean(const char *v) {
-        assert(v);
+        if (!v)
+                return -EINVAL;
 
         if (streq(v, "1") || strcaseeq(v, "yes") || strcaseeq(v, "y") || strcaseeq(v, "true") || strcaseeq(v, "t") || strcaseeq(v, "on"))
                 return 1;