fix: use default value when deserializing non-optional members (#436)

Codegen uses default values to generate the `init` for non-optional values eg. https://github.com/awslabs/aws-sdk-swift/blob/0.2.6/release/AWSSecretsManager/models/Models.swift#L4148 but while doing the deserialization, the member is considered as always present which is not true in service like AWSSecretsManager.

This change uses the the same default value is available to set the member when the response doesn't contain the field. To generate less code, codegen skips assigning nil.

Before
```
let rotationEnabledDecoded = try containerValues.decode(Swift.Bool.self, forKey: .rotationEnabled)
rotationEnabled = rotationEnabledDecoded
```
After
```
let rotationEnabledDecoded = try containerValues.decodeIfPresent(Swift.Bool.self, forKey: .rotationEnabled) ?? false
rotationEnabled = rotationEnabledDecoded
```

Author: ganeshnj

smithy-swift-codegen/src/main/kotlin/software/amazon/smithy/swift/codegen/integration/serde/json/MemberShapeDecodeGenerator.kt

@@ -20,6 +20,7 @@ import software.amazon.smithy.swift.codegen.SwiftWriter
 import software.amazon.smithy.swift.codegen.customtraits.SwiftBoxTrait
 import software.amazon.smithy.swift.codegen.integration.ProtocolGenerator
 import software.amazon.smithy.swift.codegen.integration.serde.MemberShapeDecodeGeneratable
+import software.amazon.smithy.swift.codegen.model.defaultValue
 import software.amazon.smithy.swift.codegen.model.hasTrait
 import software.amazon.smithy.swift.codegen.model.isBoxed
 import software.amazon.smithy.swift.codegen.model.recursiveSymbol
@@ -63,9 +64,14 @@ abstract class MemberShapeDecodeGenerator(
         if (member.hasTrait(SwiftBoxTrait::class.java)) {
             symbol = symbol.recursiveSymbol()
         }
-        val decodeVerb = if (symbol.isBoxed()) "decodeIfPresent" else "decode"
+        val defaultValue = symbol.defaultValue()
+        val decodeVerb = if (symbol.isBoxed() || !defaultValue.isNullOrEmpty()) "decodeIfPresent" else "decode"
         val decodedMemberName = "${memberName}Decoded"
-        writer.write("let \$L = try \$L.$decodeVerb(\$N.self, forKey: .\$L)", decodedMemberName, containerName, symbol, memberName)
+
+        // no need to assign nil to a member that is optional
+        val defaultValueLiteral = if (defaultValue != null && defaultValue != "nil") " ?? $defaultValue" else ""
+
+        writer.write("let \$L = try \$L.$decodeVerb(\$N.self, forKey: .\$L)$defaultValueLiteral", decodedMemberName, containerName, symbol, memberName)
         renderAssigningDecodedMember(member, decodedMemberName)
     }