BUG: Add null checks after ReadHeader in IPLCommonImageIO

The base class ReadHeader returns nullptr, and subclass overrides may
also return null without throwing. Two call sites in ReadImageInformation
dereference the result without null checks:

1. m_ImageHeader->modality (line 140) — null dereference when ReadHeader
   fails silently. Now throws itkExceptionMacro on null return.
2. curImageHeader->examNumber (line 223) — null dereference in the
   directory scan loop. Now skips null results with continue.

Addresses nullability.NullPassedToNonnull and core.NullDereference
findings from issue #1261.

Author: hjmjohnson

Modules/IO/IPL/src/itkIPLCommonImageIO.cxx

@@ -133,9 +133,10 @@ IPLCommonImageIO::ReadImageInformation()
   FileNameToRead = _imagePath;
 
   this->m_ImageHeader = this->ReadHeader(FileNameToRead.c_str());
-  //
-  // if anything fails in the header read, just let
-  // exceptions propagate up.
+  if (m_ImageHeader == nullptr)
+  {
+    itkExceptionMacro("ReadHeader failed for " << FileNameToRead);
+  }
 
   bool        isCT = false;
   std::string modality = m_ImageHeader->modality;
@@ -224,6 +225,10 @@ IPLCommonImageIO::ReadImageInformation()
       // throw an exception, and we'd just want to skip it.
       continue;
     }
+    if (curImageHeader == nullptr)
+    {
+      itkExceptionMacro("ReadHeader failed for " << fullPath);
+    }
     if ((((isCT) ? curImageHeader->examNumber : curImageHeader->echoNumber) == m_FilenameList->GetKey2()) &&
         (curImageHeader->seriesNumber == m_FilenameList->GetKey1()))
     {