Use default schema reload config values when config file is empty

[mattlord]

Description

A behavior change for the for SchemaReloadInterval and SchemaChangeReloadTimeout configuration options was introduced in: #14733

Specifically here, as the value for either one is an empty string after reading the empty config file: https://github.com/vitessio/vitess/pull/14733/files#diff-ee5ffc675b719e69de7147068ab4ecf03f6d5db91a000d212a30f8ce61544691R424-R431

You can see it still on main today here:

vitess/go/vt/vttablet/tabletserver/tabletenv/config.go

Lines 404 to 420 in 2e009e3

	if tmp.SchemaReloadInterval != "" {
	cfg.SchemaReloadInterval, err = time.ParseDuration(tmp.SchemaReloadInterval)
	if err != nil {
	return err
	}
	} else {
	cfg.SchemaReloadInterval = 0
	}
	if tmp.SchemaChangeReloadTimeout != "" {
	cfg.SchemaChangeReloadTimeout, err = time.ParseDuration(tmp.SchemaChangeReloadTimeout)
	if err != nil {
	return err
	}
	} else {
	cfg.SchemaChangeReloadTimeout = 0
	}

This changed the effective defaults for these two configuration options when using config files and not specifying a value for them.

The end result is that we ended up incorrectly overriding the current/default config values when parsing a config file that did NOT specify a value for SchemaReloadInterval or SchemaChangeReloadTimeout. This PR changes the behavior — to match v18 and older — so that we only override the current/default config if a value for them was specified in the config file.

This fix should be backported to v19, where the behavior change was introduced.

Related Issue(s)

• Fixes: Bug Report: v19 tablet server config parsing incorrect #16392

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

[vitess-bot]

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[codecov]

Codecov Report

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Additional details and impacted files

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