fix module manifest update logic

src/Build/Update-ModuleMeta.Tests.ps1

@@ -18,4 +18,40 @@ Describe 'Update-ModuleMeta' {
             $foo.Description[0].Text.Length | Should -BeGreaterThan 5
         }
     }
+
+    Context 'Unit Tests' -Tag 'Unit' {
+        BeforeAll {
+            # Mock Drone env if its not present
+            if (!$Env:DRONE) {
+                $Env:DRONE = $true
+                $Env:DRONE_BUILD_EVENT = 'tag'
+                $Env:DRONE_SEMVER = '9.9.9-dev9'
+                $Env:DRONE_SEMVER_SHORT = '9.9.9'
+                $Env:DRONE_SEMVER_PRERELEASE = 'dev9'
+            }
+            Mock 'Get-RepoPath' -ModuleName 'DroneHelper' {
+                return @{
+                    Src = @{
+                        Manifest = @{
+                            Item = @{
+                                FullName = './resources/MyModule.psd1'
+                            }
+                        }
+                    }
+                }
+            }
+        }
+        It 'Update test module manifest' {
+            { Update-ModuleMeta } | Should -Not -Throw
+        }
+
+        It 'Updated manifest is valid' {
+            { Test-ModuleManifest -Path './resources/MyModule.psd1' } | Should -Not -Throw
+        }
+
+        It 'External dependencies prop is untouched' {
+            $d = Test-ModuleManifest -Path './resources/MyModule.psd1'
+            $d.PrivateData.PSData.ExternalModuleDependencies.Count | Should -Be '2'
+        }
+    }
 }