parser: return error instead of panic when TZ prefix has no fields

[nghiack7]

Problem

Fixes #554

ParseStandard("TZ=0") (or any TZ=/CRON_TZ= prefix without trailing schedule fields) panics with:

panic: runtime error: slice bounds out of range [:-1]

at parser.go because strings.Index(spec, " ") returns -1 when there is no space, and the slice spec[eq+1 : -1] is invalid.

In production systems where cron expressions come from user input, this panic crashes the entire process.

Fix

One-line guard: if strings.Index(spec, " ") returns -1, return a descriptive error before touching any slice.

i := strings.Index(spec, " ")
if i < 0 {
    return nil, fmt.Errorf("timezone prefix missing schedule fields: %v", spec)
}

Tests

Added TestParseTZMissingFields in parser_test.go covering:

• "TZ=0" (invalid zone, no fields)
• "TZ=UTC" (valid zone, no fields)
• "CRON_TZ=America/New_York" (valid zone, no fields)

[nghiack7]

Friendly ping — this PR is ready for review. CI is clean ✅. Happy to make any changes requested.