Bugfix on Surface material creation and value_with_units (#3942)

* added "PhysicsTypes" property on Surface material creation.

* fixed value_with_units in case units=""

pyaedt/application/Analysis.py

@@ -2203,7 +2203,7 @@ def value_with_units(
             Some common examples are:
             "in": inches
             "cm": centimeter
-            "um":  micron
+            "um": micron
             "mm": millimeter
             "meter": meters
             "mil": 0.001 inches (mils)
@@ -2217,7 +2217,7 @@ def value_with_units(
         str
             String that combines the value and the units (e.g. "1.2mm").
         """
-        if not units:
+        if units is None:
             if unit_system == "Length":
                 units = self.modeler.model_units
             else:

pyaedt/modules/Material.py

@@ -2652,6 +2652,7 @@ def __init__(self, materiallib, name, props=None, material_update=True):
             self.physics_type = self._props["PhysicsTypes"]["set"]
         else:
             self.physics_type = ["Thermal"]
+            self._props["PhysicsTypes"] = OrderedDict({"set": ["Thermal"]})
         for property in SurfMatProperties.aedtname:
             if property in self._props:
                 mods = None