[Common][BUG-FIX] Fix SecurityConfiguration.isLegal() to return false when no security config

[jacklong319]

Purpose

Linked issue: #7146

This PR fixes a logic issue in SecurityConfiguration.isLegal() where it incorrectly returns true when no security configuration is provided, causing unnecessary security wrapper creation and checkpoint failures in Hadoop 2.7.5 environments.

Problem:

• isLegal() returns true when both keytab and principal are empty
• This causes HadoopSecuredFileSystem.trySecureFileSystem() to create unnecessary wrappers
• All file operations are wrapped with ugi.doAs(), adding overhead
• In Hadoop 2.7.5, this causes thread interruption issues during Flink checkpoints
• Results in InterruptedIOException and HDFS block replica errors

Solution:

• Change isLegal() to return false when no security configuration is provided
• Add comment explaining why we return false in this case
• Prevents HadoopSecuredFileSystem wrapper creation in non-Kerberos environments

Changes

• Changed isLegal() to return false when both keytab and principal are empty
• Added comment explaining why we return false in this case

Code Change:
// Before
return true;

// After
// Return false when no security configuration is provided to avoid
// unnecessary security wrapper creation in non-Kerberos environments
return false;