Improve handling of empty optional params

[pkoenig10]

Fixes #1331.

Before this PR

When deserializing OptionalInt, OptionalLong, or OptionalDouble values used as header or query parameters, empty string values fail to deserialize with NumberFormatException.

After this PR

When deserializing OptionalInt, OptionalLong, or OptionalDouble values used as header or query parameters, empty string values deserialize as empty optional values.

This is necessary to properly support optional header/query params for Conjure clients due to #790.

This also make the handling of primitive optional consistent with Jersey's handling of Optional<T>:
https://github.com/jersey/jersey/blob/2.25.1/core-server/src/main/java/org/glassfish/jersey/server/internal/inject/ParamConverters.java#L81-L90

Possible downsides?

• We only need this behavior for header params, but it will also apply to query params.
• This behavior violates the Conjure spec and is inconsistent with the behavior of Undertow endpoints. But Undertow endpoints have this problem for both primitive optionals and Optional<T>. See Undertow endpoints cannot deserialize optional header params sent by Feign clients #1330.

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Type

• Feature
• Improvement
• Fix
• Break
• Deprecation
• Manual task
• Migration

Description

Improve handling of empty optional params

Fixes #1331.

Before this PR

When deserializing OptionalInt, OptionalLong, or OptionalDouble values used as header or query parameters, empty string values fail to deserialize with NumberFormatException.

After this PR

When deserializing OptionalInt, OptionalLong, or OptionalDouble values used as header or query parameters, empty string values deserialize as empty optional values.

This is necessary to properly support optional header/query params for Conjure clients due to #790.

This also make the handling of primitive optional consistent with Jersey's handling of Optional<T>:
https://github.com/jersey/jersey/blob/2.25.1/core-server/src/main/java/org/glassfish/jersey/server/internal/inject/ParamConverters.java#L81-L90

Possible downsides?

• We only need this behavior for header params, but it will also apply to query params.
• This behavior violates the Conjure spec and is inconsistent with the behavior of Undertow endpoints. But Undertow endpoints have this problem for both primitive optionals and Optional<T>. See Undertow endpoints cannot deserialize optional header params sent by Feign clients #1330.
  

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[pkoenig10]

#1337 is the better fix.