clean up null checks on CameraModel/LensModel

Program.cs

@@ -144,8 +144,8 @@ void main() {
 public class Photo {
   public string File;
   public bool Loaded = false;
-  public string? CameraModel;
-  public string? LensModel;
+  public string CameraModel = "";
+  public string LensModel = "";
   public string FocalLength = "<unk>";
   public string FNumber = "<unk>";
   public string ExposureTime = "<unk>";
@@ -166,15 +166,16 @@ public class Photo {
     ExifProfile? exifs = image.Metadata.ExifProfile;
     if (exifs != null) {
       // FIXME: handle Orientation
+      // FIXME: handle date shot / edited (and sort by shot date?)
 
       IExifValue<string>? model;
       if (exifs.TryGetValue(ExifTag.Model, out model)) {
-        CameraModel = model.Value;
+        CameraModel = model.Value ?? "";
       }
 
       IExifValue<string>? lensModel;
       if (exifs.TryGetValue(ExifTag.LensModel, out lensModel)) {
-        LensModel = lensModel.Value;
+        LensModel = lensModel.Value ?? "";
       }
 
       IExifValue<Rational>? focalLength;