The necessity of null subcarrier in OFDM?
The only portion of the spectrum that is "occupied" or "transmitted" are the subcarriers that are actually used (i.e. 10 MHz instead of 15 MHz). The null subcarriers referenced in your question are used for two purposes:


The null subcarriers along the edges are used to allow for easily-realizable anti-aliasing filters. Given a sample rate of 15.36 MHz (complex), and a passband of 10 MHz, it's relatively easy to design a good, simple lowpass filter that does the job. Simple is the key here; LTE receivers are made by the millions, and any cost savings that can be achieved by design of the waveform are attractive to hardware manufacturers.
The null subcarrier at DC is used to allow the use of simple (again, cheaper) direct-conversion (zero-IF) RF receivers. One disadvantage of zero-IF receivers is that they typically encounter a strong interfering signal at DC. This can be caused by direct leakage of the LO signal into the output as well as a nonzero DC offset in the receiver's analog-to-digital converter. This is sidestepped in LTE by just not transmitting a subcarrier at the center of the band.