change default mass and charge values back to None; fix handling of N…

…one mass and charges in gsd writer

gmso/core/atom.py

@@ -39,12 +39,12 @@ class Atom(Site):
         the gmso.abc.abstract site class
     """
     charge_: Optional[Union[u.unyt_quantity, float]] = Field(
-        0.0,
-        description="Charge of the atom. Defaults to 0.0 if not set.",
+        None,
+        description="Charge of the atom.",
     )
 
     mass_: Optional[Union[u.unyt_quantity, float]] = Field(
-        0.0, description="Mass of the atom. Defaults to 0.0 if not set."
+        None, description="Mass of the atom."
     )
 
     element_: Optional[Element] = Field(

gmso/formats/gsd.py

@@ -128,9 +128,11 @@ def _write_particle_information(
 
     masses = np.array([site.mass for site in top.sites])
     masses[masses == 0] = 1.0
+    masses[masses == None] = 1.0
     gsd_snapshot.particles.mass = masses / ref_mass
 
     charges = np.array([site.charge for site in top.sites])
+    charges[charges == None] = 0.0
     e0 = u.physical_constants.eps_0.in_units(
         u.elementary_charge**2 / u.Unit("kcal*angstrom/mol")
     )