fix: Prevent invalid syntax for no-op .replace ops

Author: TrevorBergeron

bigframes/core/compile/ibis_compiler/scalar_op_registry.py

@@ -1173,6 +1173,10 @@ def udf(*inputs):
 
 @scalar_op_compiler.register_unary_op(ops.MapOp, pass_op=True)
 def map_op_impl(x: ibis_types.Value, op: ops.MapOp):
+    # this should probably be handled by a rewriter
+    if len(op.mappings) == 0:
+        return x
+
     case = ibis_api.case()
     for mapping in op.mappings:
         case = case.when(x == mapping[0], mapping[1])

tests/system/small/test_series.py

@@ -733,10 +733,12 @@ def test_series_replace_nans_with_pd_na(scalars_dfs):
     (
         ({"Hello, World!": "Howdy, Planet!", "T": "R"},),
         ({},),
+        ({0: "Hello, World!"},),
     ),
     ids=[
         "non-empty",
         "empty",
+        "off-type",
     ],
 )
 def test_series_replace_dict(scalars_dfs, replacement_dict):