Tentative fix of subsec to `Time.at` [Bug #18293]

time.c

@@ -2683,7 +2683,7 @@ time_s_at(rb_execution_context_t *ec, VALUE klass, VALUE time, VALUE subsec, VAL
     VALUE t;
     wideval_t timew;
 
-    if (!NIL_P(subsec)) {
+    if (subsec) {
         int scale = get_scale(unit);
         time = num_exact(time);
         t = num_exact(subsec);

timev.rb

@@ -267,7 +267,7 @@ class Time
   # :include: doc/time/nsec.rdoc
   # :include: doc/time/in.rdoc
   #
-  def self.at(time, subsec = nil, unit = :microsecond, in: nil)
+  def self.at(time, subsec = false, unit = :microsecond, in: nil)
     __builtin.time_s_at(time, subsec, unit, __builtin.arg!(:in))
   end