chore(csharp/test/Drivers/Databricks): Fix some test cases that have null catalog (#3139)

## Issue 

In some test cases in `DatabricksConnectionTest` and `StatementTest` in
Databricks driver test, when catalog/schema is `null`, the test code
does not update the corresponding catalog/schema in `TestConfiguration`,
if the `catalog` or `schema` is non-empty in the original
`TestConfiguration`, the test case will fail

## Changes
- Updated
`DatabricksConnectionTest:SetDefaultCatalogAndSchemaOptionsTest` to set
catalog/schema in testConfig to `string.empty` when catalog/schema is
null
- Updated `StatementTest:ShouldReturnEmptyPkFkResult_WorksAsExpected` to
set statement's catalog to `string.empty` when it is null

## Testing
- `Apache.Arrow.Adbc.Tests.Drivers.Databricks.DatabricksConnectionTest`
- `Apache.Arrow.Adbc.Tests.Drivers.Databricks.StatementTest`

Author: jackyhu-db

csharp/test/Drivers/Databricks/E2E/DatabricksConnectionTest.cs

@@ -373,8 +373,22 @@ public async Task SetDefaultCatalogAndSchemaOptionsTest(string? inputCatalog, st
             var testConfig = (DatabricksTestConfiguration)TestConfiguration.Clone();
             testConfig.EnableMultipleCatalogSupport = enableMultipleCatalogSupport;
 
-            if (inputCatalog is not null) testConfig.Catalog = inputCatalog;
-            if (inputSchema is not null) testConfig.DbSchema = inputSchema;
+            if (inputCatalog is not null)
+            {
+                testConfig.Catalog = inputCatalog;
+            }
+            else
+            {
+                testConfig.Catalog = string.Empty;
+            }
+            if (inputSchema is not null)
+            {
+                testConfig.DbSchema = inputSchema;
+            }
+            else
+            {
+                testConfig.DbSchema = string.Empty;
+            }
 
             var connection = NewConnection(testConfig);
             var statement = connection.CreateStatement();

csharp/test/Drivers/Databricks/E2E/StatementTests.cs

@@ -1123,6 +1123,10 @@ public void ShouldReturnEmptyPkFkResult_WorksAsExpected(bool enablePKFK, string?
             {
                 statement.SetOption(ApacheParameters.CatalogName, catalogName);
             }
+            else
+            {
+                statement.SetOption(ApacheParameters.CatalogName, string.Empty);
+            }
 
             // Act
             var result = ((DatabricksStatement)statement).ShouldReturnEmptyPkFkResult();