Fixed bug in install_sdp.sh config loading that caused P4ADMINPASS
to be silently cleared when a setting was absent from the user's config file.
The config loading loop unconditionally overwrote all Config array defaults
with empty string for any key not present in the user's config file. This
caused the Password default to be erased if the user's sdp_install.cfg did
not explicitly include a Password= entry. The resulting empty P4ADMINPASS
was written into the generated mkdirs.N.cfg, leading to the error:
Failed to decode P4ADMINPASS from base64 form: [].
The fix guards the assignment so that defaults are preserved for any key
not found in the config file.