feat: Make empty string handling for optional params consistent

[FranciscoKurpiel]

This change introduces consistent behavior across the library for how optional parameters with default values handle empty strings ("") provided by configuration sources.

Previously, the behavior was inconsistent:

• string and xtypes.String would accept "" as a valid value, overriding the default.
• xtypes.URL would also override the default with an empty URL.
• Most other basic types (int, bool, time.Duration, etc.) and xtypes (xtypes.Bool, xtypes.Integer) would fail with a parsing error.

This commit aligns all non-string types to a single, predictable behavior.

Behavior Change:

For all optional parameters except string and xtypes.String, providing an empty string from a configuration source is now treated as an "absent" value. This causes the parameter to correctly fall back to its specified default value.

• Affected Types: time.Time, time.Duration, bool, all numeric types (int, uint, etc.), xtypes.URL, xtypes.Bool, xtypes.Integer, and xtypes.RawJSON.
• Unaffected Types: string and xtypes.String continue to treat "" as a valid, intentional value.

Implementation Details:

• The validFn for the affected basic types and ValueValid for the affected xtypes were updated to return types.ErrNoValue when given an empty string. The parser uses this signal to ignore the value for optional parameters or error out for required ones.
• The TestEmptyURL test was updated to reflect this new, stricter validation for required parameters.
• New tests (TestOptionalBasicTypes and TestOptionalXTypes) were added to parser_test.go to provide comprehensive coverage for this behavior.
• The README.md was updated to document this behavior for users.

[gemini-code-assist]

Summary of Changes

Hello @FranciscoKurpiel, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a significant improvement in the library's configuration parsing by ensuring consistent and predictable behavior when optional parameters receive empty string values from configuration sources. The change aligns the handling of empty strings for most non-string types, allowing them to correctly utilize their default values instead of causing parsing errors or unintended overrides. This enhances the robustness and user-friendliness of the configuration system.

Highlights

• Consistent Empty String Handling: Standardized how optional parameters with default values handle empty strings ("") provided by configuration sources across the library.
• Behavior Change for Non-String Types: For most non-string types (e.g., int, bool, time.Duration, xtypes.URL, xtypes.Bool, xtypes.Integer, xtypes.RawJSON), an empty string is now treated as an "absent" value, causing the parameter to fall back to its specified default value.
• String Types Unaffected: string and xtypes.String types continue to treat "" as a valid, intentional value that overrides any default.
• Implementation Details: The validFn for basic types and ValueValid for xtypes were updated to return types.ErrNoValue when an empty string is encountered, signaling the parser to ignore the value for optional parameters.
• Testing and Documentation: New comprehensive tests (TestOptionalBasicTypes, TestOptionalXTypes) were added, TestEmptyURL was updated, and the README.md was modified to document this new behavior.

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[gemini-code-assist]

Code Review

This pull request introduces a valuable change by making the handling of empty strings for optional parameters consistent across the library. The approach of treating "" as an absent value for non-string types, causing them to fall back to their defaults, is a significant improvement in predictability and usability. The changes are well-implemented, and the addition of comprehensive tests in parser_test.go and updates to xtypes/url_test.go ensure the new behavior is well-covered. The documentation update in README.md is also clear and helpful for users. I have one suggestion regarding code duplication that could improve maintainability.