How to reproduce:
1. Open the attached "1317735 repro" project
2. Open the "SampleScene" Scene from the Assets folder
3. Select the "GameObject" GameObject from the Hierarchy window
4. Expand the "Events" dropdown of the Player Input component

Expected results: the list is expended without any errors and the 3rd Action Map is shown
Actual results: multiple errors are thrown to the Console window and only the first Action Map is shown

Reproducible with: 1.0.0, 1.0.2, 1.1.0-preview.3 (2019.4.22f1, 2020.2.6f1, 2021.1.0b9, 2021.2.0a7)

Notes:
-The issue occurs because the second Action Map "New action map1" doesn't have any Actions associated with it
-Full error message:
Exception: Style.Draw may not be called with GUIContent that is null.
UnityEngine.GUIStyle.Draw (UnityEngine.Rect position, UnityEngine.GUIContent content, System.Int32 controlId, System.Boolean isHover, System.Boolean isActive, System.Boolean on, System.Boolean hasKeyboardFocus) (at <f8197c5efb504ab8a77daf7d0e9abc5e>:0)
UnityEngine.GUIStyle.Draw (UnityEngine.Rect position, UnityEngine.GUIContent content, System.Int32 controlID, System.Boolean on) (at <f8197c5efb504ab8a77daf7d0e9abc5e>:0)
UnityEditor.EditorGUI.FoldoutInternal (UnityEngine.Rect position, System.Boolean foldout, UnityEngine.GUIContent content, System.Boolean toggleOnLabelClick, UnityEngine.GUIStyle style) (at <8c48e4bc2ff34638b69095cbb65e2e27>:0)
UnityEditor.EditorGUI.Foldout (UnityEngine.Rect position, System.Boolean foldout, UnityEngine.GUIContent content, System.Boolean toggleOnLabelClick, UnityEngine.GUIStyle style) (at <8c48e4bc2ff34638b69095cbb65e2e27>:0)
UnityEditor.EditorGUILayout.FoldoutInternal (System.Boolean foldout, UnityEngine.GUIContent content, System.Boolean toggleOnLabelClick, UnityEngine.GUIStyle style) (at <8c48e4bc2ff34638b69095cbb65e2e27>:0)
UnityEditor.EditorGUILayout.Foldout (System.Boolean foldout, UnityEngine.GUIContent content, System.Boolean toggleOnLabelClick, UnityEngine.GUIStyle style) (at <8c48e4bc2ff34638b69095cbb65e2e27>:0)
UnityEditor.EditorGUILayout.Foldout (System.Boolean foldout, UnityEngine.GUIContent content, System.Boolean toggleOnLabelClick) (at <8c48e4bc2ff34638b69095cbb65e2e27>:0)
UnityEngine.InputSystem.Editor.PlayerInputEditor.OnInspectorGUI () (at Library/PackageCache/com.unity.inputsystem@1.0.2/InputSystem/Plugins/PlayerInput/PlayerInputEditor.cs:186)
UnityEditor.UIElements.InspectorElement+<>c__DisplayClass59_0.<CreateIMGUIInspectorFromEditor>b__0 () (at <420b942f64794d4da66420603f126f7b>:0)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr, Boolean&)