Guard against empty SxExpr in _as_sx and _build_component_ast

Fragment responses with text/sx content-type but empty body create
SxExpr(""), which is truthy but fails to parse. Handle this by
returning None from _as_sx for empty SxExpr sources, and treating
empty SxExpr as NIL in _build_component_ast.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/helpers.py

@@ -77,7 +77,7 @@ def _as_sx(val: Any) -> SxExpr | None:
     if not val:
         return None
     if isinstance(val, SxExpr):
-        return val
+        return val if val.source else None
     html = str(val)
     escaped = html.replace("\\", "\\\\").replace('"', '\\"')
     return SxExpr(f'(~rich-text :html "{escaped}")')
@@ -376,7 +376,10 @@ def _build_component_ast(__name: str, **kwargs: Any) -> list:
         elif isinstance(val, SxExpr):
             # SxExpr values need to be parsed into AST
             from .parser import parse
-            ast.append(parse(val.source))
+            if not val.source:
+                ast.append(NIL)
+            else:
+                ast.append(parse(val.source))
         else:
             ast.append(val)
     return ast