Fix NPE on trait with implicit null

[srchase]

This PR fixes smithy-syntax throwing a NullPointerException when encountering a trait with an implicit null.

Following this fix, if a trait is encountered with an empty body, the parentheses will be omitted.

This:

structure Struct {
    @default()
    a: String

    @default( )
    b: String

    @default(null)
    c: String
}

Will now be formatted to this:

structure Struct {
    @default
    a: String

    @default
    b: String

    @default(null)
    c: String
}

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[mtdowling]

$version: "2"
namespace smithy.example

@trait
structure foo {}

@foo
string MyString1

@foo()
string MyString2

@foo(null)
string MyString3

Results in:

──  ERROR  ────────────────────────────────────────────────────────── TraitValue
Shape: smithy.example#MyString3
File:  b.smithy:13:1

13| @foo(null)
  | ^
14| string MyString3

Error validating trait smithy.example#foo: Expected object value for structure
shape, smithy.example#foo; found null value

I think the only reliable approach is to omit () in the formatter:

@foo()

Becomes

@foo

[mtdowling]

See comment about this creating invalid models