fix: handle empty string delta.content in OpenAI streaming

Some providers send an empty string as the first delta to signal
streaming start. The falsy check `if (delta.content)` treated "" as
absent, skipping content_block_start. The next delta with actual
content was emitted without it, violating the Anthropic protocol.

Changed to `delta.content != null` to distinguish between absent field
and empty string.

Relates to #42

Co-Authored-By: Juan Camilo <juancamilo.auriti@gmail.com>

src/services/api/openaiShim.ts

@@ -345,8 +345,9 @@ async function* openaiStreamToAnthropic(
       for (const choice of chunk.choices ?? []) {
         const delta = choice.delta
 
-        // Text content
+        // Text content — use != null to distinguish absent field from empty string,
-        if (delta.content) {
+        // some providers send "" as first delta to signal streaming start
+        if (delta.content != null) {
           if (!hasEmittedContentStart) {
             yield {
               type: 'content_block_start',