[SPARK-8200] [MLLIB] Check for empty RDDs in StreamingLinearAlgorithm

Test cases for both StreamingLinearRegression and StreamingLogisticRegression, and code fix.

Edit:
This contribution is my original work and I license the work to the project under the project's open source license.

Author: Paavo <pparkkin@gmail.com>

Closes #6713 from pparkkin/streamingmodel-empty-rdd and squashes the following commits:

ff5cd78 [Paavo] Update strings to use interpolation.
db234cf [Paavo] Use !rdd.isEmpty.
54ad89e [Paavo] Test case for empty stream.
393e36f [Paavo] Ignore empty RDDs.
0bfc365 [Paavo] Test case for empty stream.