Selectively substituting null data with a constant value in a raster

[asmitashukla]

 Raster with null data

Hello - I am using the raster domain tool to create a polygon feature representing the extent of the raster. I first used the set null tool to eliminate all no data pixels. That got rid of the white box around the raster but also created gaps in between. Now when I run the raster domain tool, the resulting polygon has a small gaps where pixels with null data existed (image below). Is there a way to fill these small gaps or some how get rid of only those pixels which have null values outside the extent of the raster with data (white space outside the actual raster I am interested in). I am new to ArcPro and  any help is greatly appreciated. Thanks!