Revert "fix the kokoro issue with unreliable row count"

This reverts commit e3180f7.

Author: shuoweil

bigframes/core/rewrite/slices.py

@@ -77,12 +77,15 @@ def rewrite_slice(node: nodes.BigFrameNode):
 
     slice_def = (node.start, node.stop, node.step)
 
-    # Handle empty slice cases explicitly before any row count dependent logic
-    # This ensures empty slices always return empty results regardless of statistics
-    if _is_empty_slice(node.start, node.stop, node.step):
-        # Create a filter that will always return empty results
-        # Use start=0, stop=0, step=1 to ensure empty result
-        return slice_as_filter(node.child, 0, 0, 1)
+    # Handle empty slice cases exlicitly (e.g. [0:0])
+    if (
+        node.start is not None
+        and node.stop is not None
+        and node.start >= node.stop
+        and (node.step is None or node.step > 0)
+    ):
+        # Return empty result by filtering with impossible condition
+        return slice_as_filter(node.child, node.start, node.start, node.step or 1)
 
     # no-op (eg. df[::1])
     if slices.is_noop(slice_def, node.child.row_count):
@@ -97,26 +100,6 @@ def rewrite_slice(node: nodes.BigFrameNode):
     return slice_as_filter(node.child, *slice_def)
 
 
-def _is_empty_slice(start, stop, step):
-    """Check if a slice will always return empty results."""
-    if start is None or stop is None:
-        return False
-
-    # Normalize step
-    if step is None:
-        step = 1
-
-    # For positive step, empty if start >= stop
-    # For negative step, empty if start <= stop
-    if step > 0:
-        return start >= stop
-    elif step < 0:
-        return start <= stop
-    else:
-        # step == 0 is invalid, but handle gracefully
-        return True
-
-
 def slice_as_filter(
     node: nodes.BigFrameNode, start: Optional[int], stop: Optional[int], step: int
 ) -> nodes.BigFrameNode: