- Where is the SPFILE for a PDB database ?

PDBs do not have an init.ora file or SPFILE,only the root's CDB has one. Parameters for the PDBs are stored in the DB's dictionary,in the PDB_SPFILE$ table.

- I'm querying the PDB's PDB_SPFILE$ table and it is empty,why ?

This is expected,all the information about the modified parameters for a PDB are stored in the PDB_SPFILE$ table in the ROOT CDB.
The only time when parameters are copied into the PDB's own pdb_spfile$ table is during PDB UNPLUG,and this is done as a fallback in case the XML is lost.
Once the PDB plug back into another CDB the PDB's PDB_SPFILE$ table is truncated.