Fix null modal-content-color in modal.scss (twbs#39977)

Identified that $modal-content-color was set to null, which could lead
to display issues in React Bootstrap Modals. Changed the value to --#{$prefix}body-color
to ensure consistency and correct color application in modal components.

Author: confidential-nt

Diff for: scss/_variables.scss

@@ -1505,7 +1505,7 @@ $modal-dialog-margin-y-sm-up:       1.75rem !default;
 
 $modal-title-line-height:           $line-height-base !default;
 
-$modal-content-color:               null !default;
+$modal-content-color:               var(--#{$prefix}body-color) !default;
 $modal-content-bg:                  var(--#{$prefix}body-bg) !default;
 $modal-content-border-color:        var(--#{$prefix}border-color-translucent) !default;
 $modal-content-border-width:        var(--#{$prefix}border-width) !default;