fix integration install location unit test

- testing with custom vendor location empty and maya location not empty was failing because if custom vendor location is empty it would return the files in the default install location, which vary by machine.
- instead only look for default vendor location if environment variable does not exist

Author: vkovec

Assets/com.unity.formats.fbx.tests/FbxExportSettingsTest.cs

@@ -482,7 +482,7 @@ private void TestLocations(Dictionary<string,List<string>> data)
             ExportSettings.instance.ClearDCCOptions();
 
             GUIContent[] options = ExportSettings.GetDCCOptions();
-            Assert.AreEqual(options.Length, expectedResult);
+            Assert.AreEqual(expectedResult, options.Length);
 
             if (data.ContainsKey("expected3DApp"))
             {

Packages/com.unity.formats.fbx/Editor/Scripts/FbxExportSettings.cs

@@ -283,12 +283,13 @@ private static string GetMayaLocationFromEnvironmentVariable(string env)
         /// </summary>
         private static HashSet<string> GetCustomVendorLocations()
         {
-            HashSet<string> result = new HashSet<string>();
+            HashSet<string> result = null;
 
             var environmentVariable = Environment.GetEnvironmentVariable("UNITY_3DAPP_VENDOR_LOCATIONS");
 
             if (!string.IsNullOrEmpty(environmentVariable))
             {
+                result = new HashSet<string>();
                 string[] locations = environmentVariable.Split(';');
                 foreach (var location in locations)
                 {
@@ -298,8 +299,6 @@ private static HashSet<string> GetCustomVendorLocations()
                     }
                 }
             }
-            
-
             return result;
         }
 
@@ -347,7 +346,7 @@ public static string[] DCCVendorLocations
             {
                 HashSet<string> result = GetCustomVendorLocations();
 
-                if (result == null || result.Count < 1)
+                if (result == null)
                 {
                     result = GetDefaultVendorLocations();
                 }