If they only made diamonds more common it would have no effect on strongholds or any other structures, and if strongholds had been disabled Eyes of Ender would literally do nothing (not leave your hand when right-clicked).
You could try finding another stronghold, which requires that you travel quite far since in 1.9+ they are at least 1408 blocks away from the origin (0,0) at 120 degree angles (for the first three; there are a total of 128). I'm betting that you generated the chunks you "found" the stronghold in in 1.8 and did not look for it until now and the game thinks there should be one based on the current version (though it should have recorded the locations of the 1.8 strongholds so it can still find them; likewise, 1.6.4 was released so their locations were remembered after upgrading to 1.7). To find another stronghold try going north or southeast (about 120 degrees away) starting at the origin and Eyes of Ender should start pointing to one of them once you are no more than 600-700 blocks away (presuming they are at the maximum allowed distance, which is up to 2688 blocks away from the origin or 4134 from the one you found; this means that you need to travel up to 2067 blocks before getting closer to another). The second ring of strongholds is quite a bit further away, at least 4480 blocks, with the rest as far as 24192 blocks away, and I doubt anybody has ever explored the entire region out to that point.
Also, if you did start the world in 1.8 and don't mind cheating a bit you could look up the locations of the original three strongholds, which are 640-1152 blocks away and may have been generated if you explored enough to generate terrain where the 1.9+ strongholds would be*. Remember, world type does matter, which is why the link includes Large Biomes as a separate option (note that there are no options for custom biome sizes but a Customized world with a biome size of 4 is the same as Default and 6 is Large Biomes).
*Here is a comparison between 1.7-1.8 and 1.9+, both set to default; notice that the strongholds in 1.7-1.8 easily fit within the area of the closest 3 in 1.9+, and would also be similar to your world because the stronghold to the southwest has a similar angle as the one you found, hence why I said to go north or southeast. Also, a stronghold located on the second ring may be closer to the one you found, but further from the origin:
Are you sure that your world was created in 1.9 or later? They changed the locations of strongholds in 1.9 and that is the only reason I can think of for there not to be one where Eyes of Ender take you, besides using a world type different from default, as their locations are partially dependent on biome layout (if you accidentally disabled them when making a custom world Eyes of Ender would not work at all).