Account for missing system symbol table (amazon-ion#953)

Fixes amazon-ion#952

src/main/java/com/amazon/ion/impl/LocalSymbolTableImports.java

@@ -242,13 +242,15 @@ SymbolTable getSystemSymbolTable()
      */
     SymbolTable[] getImportedTables()
     {
-        int count = myImports.length - 1; // we don't include system symtab
-        SymbolTable[] imports = new SymbolTable[count];
-        if (count > 0)
-        {
-            // defensive copy
-            System.arraycopy(myImports, 1, imports, 0, count);
-        }
+        // We have only the system symbol table, or we have none.
+        // None implies an empty system symbol table, as in Ion 1.1.
+        if (myImports.length == 1 || myImports.length == 0) return new SymbolTable[0];
+
+        int nonSystemTables = myImports.length - 1; // we don't include system symtab
+
+        SymbolTable[] imports = new SymbolTable[nonSystemTables];
+        // defensive copy
+        System.arraycopy(myImports, 1, imports, 0, nonSystemTables);
         return imports;
     }
 

src/test/java/com/amazon/ion/impl/LocalSymbolTableImportsTest.kt

@@ -0,0 +1,14 @@
+// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+package com.amazon.ion.impl
+
+import org.hamcrest.MatcherAssert.assertThat
+import org.hamcrest.Matchers
+import org.junit.jupiter.api.Test
+
+internal class LocalSymbolTableImportsTest {
+    @Test
+    fun `EMPTY#getImportedTables should be empty`() {
+        assertThat(LocalSymbolTableImports.EMPTY.importedTables, Matchers.emptyArray())
+    }
+}