Fix Runnable.transform() for false-y inputs (#10893)

---------

Co-authored-by: Bagatur <baskaryan@gmail.com>

libs/langchain/langchain/schema/runnable/base.py

@@ -286,16 +286,19 @@ class Runnable(Generic[Input, Output], ABC):
         Subclasses should override this method if they can start producing output while
         input is still being generated.
         """
-        final: Union[Input, None] = None
+        final: Input
+        got_first_val = False
 
         for chunk in input:
-            if final is None:
+            if not got_first_val:
                 final = chunk
+                got_first_val = True
             else:
                 # Make a best effort to gather, for any type that supports `+`
                 # This method should throw an error if gathering fails.
                 final += chunk  # type: ignore[operator]
-        if final:
+
+        if got_first_val:
             yield from self.stream(final, config, **kwargs)
 
     async def atransform(
@@ -309,17 +312,19 @@ class Runnable(Generic[Input, Output], ABC):
         Subclasses should override this method if they can start producing output while
         input is still being generated.
         """
-        final: Union[Input, None] = None
+        final: Input
+        got_first_val = False
 
         async for chunk in input:
-            if final is None:
+            if not got_first_val:
                 final = chunk
+                got_first_val = True
             else:
                 # Make a best effort to gather, for any type that supports `+`
                 # This method should throw an error if gathering fails.
                 final += chunk  # type: ignore[operator]
 
-        if final:
+        if got_first_val:
             async for output in self.astream(final, config, **kwargs):
                 yield output