fix(python): panic on empty pip_binary in config

Signed-off-by: innocentzero <isfarulhaque@proton.me>

crates/pacdef_core/src/config.rs

@@ -25,7 +25,7 @@ pub struct Config {
     #[serde(default)]
     pub disabled_backends: Vec<String>,
     /// Choose whether to use pipx instead of pip for python package management
-    #[serde(default)]
+    #[serde(default = "pip")]
     pub pip_binary: String,
 }
 
@@ -37,6 +37,10 @@ fn aur_helper() -> String {
     "paru".into()
 }
 
+fn pip() -> String {
+    "pip".into()
+}
+
 impl Config {
     /// Load the config from the associated file.
     ///