Fix Accessors getters for nullable string/octstr attributes.

Callers have to pass a Nullable<Span> that already points to the buffer to
fill.  But we were calling SetNonNull(), which reset the Nullable to point to an
empty span, after which copying the data in of course failed.

The right thing to do is to ensure that the Nullable has a value, then use that
value.

Author: bzbarsky-apple

src/app/zap-templates/templates/app/attributes/Accessors-src.zapt

@@ -14,6 +14,7 @@
 #include <app/util/attribute-storage-null-handling.h>
 #include <app/util/odd-sized-integers.h>
 #include <lib/core/CHIPEncoding.h>
+#include <lib/support/logging/CHIPLogging.h>
 
 namespace chip {
 namespace app {
@@ -38,6 +39,13 @@ namespace {{asUpperCamelCase label}} {
 Protocols::InteractionModel::Status Get(chip::EndpointId endpoint, {{accessorGetterType this}} value)
 {
     {{~#if (isString type)}}
+    {{#if isNullable}}
+    if (value.IsNull()) {
+      ChipLogError(Zcl, "Null Nullable<Span> passed to {{asUpperCamelCase parent.label}}::{{asUpperCamelCase label}}::Get");
+      return Protocols::InteractionModel::Status::Failure;
+    }
+
+    {{/if}}
     {{~#*inline "lengthType"}}uint{{#if (isShortString type)}}8{{else}}16{{/if}}_t{{/inline}}
     uint8_t zclString[{{maxLength}} + {{>sizingBytes}}];
     Protocols::InteractionModel::Status status = emberAfReadAttribute(endpoint, {{>clusterId}}, Id, zclString, sizeof(zclString));
@@ -53,7 +61,7 @@ Protocols::InteractionModel::Status Get(chip::EndpointId endpoint, {{accessorGet
       {{/if}}
     }
     {{#if isNullable}}
-    auto & span = value.SetNonNull();
+    auto & span = value.Value();
     {{/if}}
     {{~#*inline "value"}}{{#if isNullable}}span{{else}}value{{/if}}{{/inline}}
     VerifyOrReturnError({{>value}}.size() == {{maxLength}}, Protocols::InteractionModel::Status::InvalidDataType);