fix: ignore nil or empty $FacebookSDKVersion override

Guard the override on a non-empty value so a nil/blank
$FacebookSDKVersion (e.g. from a missing env var) falls back to the
default `>= 17.0.2, < 19.0` range instead of dropping the version cap.

Scanned-by: gitleaks 8.29.0

Author: vgupta98

Rudder-Facebook.podspec

@@ -25,7 +25,7 @@ Pod::Spec.new do |s|
     s.source_files = 'Rudder-Facebook/Classes/**/*'
     s.ios.deployment_target = deployment_target
 
-    if defined?($FacebookSDKVersion)
+    if defined?($FacebookSDKVersion) && !$FacebookSDKVersion.to_s.empty?
       facebook_sdk_version = $FacebookSDKVersion
       Pod::UI.puts "#{s.name}: Using user specified Facebook SDK version '#{$FacebookSDKVersion}'"
     else