Empty tooltip should not be shown

[subarroca]

What is the expected behavior?

If tooltip is undefined, null or '' no tooltip should be displayed

What is the current behavior?

Empty tooltip shown

What are the steps to reproduce?

add [md-tooltip] = null to any tag and hover it

Which versions of Angular, Material, OS, browsers are affected?

@ angular 2.2.4, @ angular/material 2.0.0-alpha.10

[subarroca]

Thanks!

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