[BUG] Conditional join with empty left table throws exception

[jlowe]

Describe the bug
Performing a conditional join when the left table has zero rows throws the following exception:

cuDF failure at: ../include/cudf/detail/utilities/cuda.cuh:65: num_blocks must be > 0

Steps/Code to reproduce bug
Apply the following patch and run JOIN_TEST:

diff --git a/cpp/tests/join/conditional_join_tests.cu b/cpp/tests/join/conditional_join_tests.cu
index e16e1ec7de..ad70fe9a46 100644
--- a/cpp/tests/join/conditional_join_tests.cu
+++ b/cpp/tests/join/conditional_join_tests.cu
@@ -563,6 +563,16 @@ TYPED_TEST(ConditionalFullJoinTest, TestOneColumnNoneEqual)
               {JoinNoneValue, 2}});
 };
 
+TYPED_TEST(ConditionalFullJoinTest, TestOneColumnLeftEmpty)
+{
+  this->test({{}},
+             {{3, 4, 5}},
+             left_zero_eq_right_zero,
+             {{JoinNoneValue, 0},
+              {JoinNoneValue, 1},
+              {JoinNoneValue, 2}});
+};
+
 TYPED_TEST(ConditionalFullJoinTest, TestTwoColumnThreeRowSomeEqual)
 {
   this->test({{0, 1, 2}, {10, 20, 30}},

Note that this fails for other join types as well such as left and inner joins.

Expected behavior
Joins should not crash when given one or more empty tables as input.