reject empty or whitespace timedelta values in _parse_timedelta

[HrachShah]

What

_parse_timedelta in tornado/options.py silently accepted empty ("") and whitespace-only (" ", "\t", "\n") values as datetime.timedelta(0). Every other type-specific parser in the same module (_parse_datetime, _parse_bool, _parse_string) rejects empty input with options.Error, so this is inconsistent.

Passing --t= or --t=" " to an application using tornado.options would set a 0-second timeout instead of raising the user-visible error that the rest of the module produces.

Repro

from tornado.options import OptionParser
import datetime
opts = OptionParser()
opts.define("t", type=datetime.timedelta)
opt = opts._options["t"]
assert opt._parse_timedelta("") == datetime.timedelta(0)   # silently accepted

Fix

Early-exit guard at the top of _parse_timedelta:

if not value or not value.strip():
    raise Error("Invalid time delta: %r" % value)

The existing inner try: ... except Exception: raise is left as-is — it still legitimately catches float() conversion and timedelta(**{units: num}) construction errors. This commit focuses narrowly on the silent-accept bug; a separate PR (the existing fix/options-parse-timedelta-options-error branch) was already in flight to clean up the raise Exception() / raise no-op pair.

Test

test_parse_timedelta_empty_or_whitespace_raises constructs an _Option directly and calls _parse_timedelta(bad) for bad in ("", " ", "\t", "\n", " \t "), asserting options.Error is raised with the offending value in the message. The test fails on the pre-fix code (the call returns timedelta(0) instead of raising).

Verified

• python3 -m pytest tornado/test/options_test.py -v — 25 passed, 0 failures
• Pre-fix git stash + test rerun shows the regression test fails as expected
• All existing options tests still pass (test_types, test_types_with_conf_file, test_parse_callbacks, etc.)