Update IntelliJPluginSpec test for empty plugin ID

Ensure the "idea.required.plugins.id" property is set to an empty string instead of not being present in the test command properties. This change helps in maintaining consistency and avoids potential issues when expecting the property to be present.

Author: hsz

src/test/kotlin/org/jetbrains/intellij/IntelliJPluginSpec.kt

@@ -46,7 +46,7 @@ class IntelliJPluginSpec : IntelliJPluginSpecBase() {
             val testCommand = parseCommand(it.output)
 
             assertPathParameters(testCommand, sandboxPath)
-            assertFalse(testCommand.properties.containsKey("idea.required.plugins.id"))
+            assertEquals(testCommand.properties.get("idea.required.plugins.id"), "")
 
             assertEquals("boot.jar", File(testCommand.xclasspath).name)
             assertEquals("lib", File(testCommand.xclasspath).parentFile.name)