fix(time): unix() and unix_nano() returns 0 for epoch

Time zero value is "0001-01-01T00:00:00Z". Tests expected zero
value have unix()/unix_nano() to be 0, but this is only the case
for epoch.

time/testdata/test.star

@@ -27,7 +27,7 @@ assert.eq(10.0, d2.hours())
 assert.eq(10*60.0, d2.minutes())
 assert.eq(10*60*60.0, d2.seconds())
 assert.eq(10*60*60*1000000000, d2.nanoseconds())
-assert.eq(time.zero.unix(), 0)
-assert.eq(time.zero.unix_nano(), 0)
+assert.eq(time.time("1970-01-01T00:00:00Z").unix(), 0)
+assert.eq(time.time("1970-01-01T00:00:00Z").unix_nano(), 0)
 
 time.sleep(time.second)